Can't upgrade to 1.2

[picks44]

I was using v0.3.1 from now on (started with a Symfony 2.8 at the time), and now I wanted to upgrade to v1.2, while my Symfony version is 3.4.
But the composer require refuse to do it.
Could you help?

$ composer require cspoo/swiftmailer-mailgun-bundle --update-with-dependencies
Using version ^1.2 for cspoo/swiftmailer-mailgun-bundle
./composer.json has been updated
Dependency "symfony/symfony" is also a root requirement, but is not explicitly whitelisted. Ignoring.
Dependency "symfony/symfony" is also a root requirement, but is not explicitly whitelisted. Ignoring.
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: remove swiftmailer/swiftmailer v5.4.12
    - Conclusion: don't install swiftmailer/swiftmailer v6.2.3
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.12
    - Conclusion: don't install swiftmailer/swiftmailer v6.2.2
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.11
    - Conclusion: don't install swiftmailer/swiftmailer v6.2.1
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.10
    - Conclusion: don't install swiftmailer/swiftmailer v6.2.0
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.9
    - Conclusion: don't install swiftmailer/swiftmailer v6.1.3
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.8
    - Conclusion: don't install swiftmailer/swiftmailer v6.1.2
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.7
    - Conclusion: don't install swiftmailer/swiftmailer v6.1.1
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.6
    - Conclusion: don't install swiftmailer/swiftmailer v6.1.0
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.5
    - Conclusion: don't install swiftmailer/swiftmailer v6.0.2
    - Conclusion: don't install swiftmailer/swiftmailer v5.4.4
    - Installation request for cspoo/swiftmailer-mailgun-bundle ^1.2 -> satisfiable by cspoo/swiftmailer-mailgun-bundle[1.2.0].
    - symfony/swiftmailer-bundle v2.6.7 requires swiftmailer/swiftmailer ~4.2|~5.0 -> satisfiable by swiftmailer/swiftmailer[v5.4.12, v4.2.0, v4.2.1, v4.2.2, v4.3.0, v4.3.1, v5.0.0, v5.0.1, v5.0.2, v5.0.3, v5.1.0, v5.2.0, v5.2.1, v
5.2.2, v5.3.0, v5.3.1, v5.4.0, v5.4.1, v5.4.10, v5.4.11, v5.4.2, v5.4.3, v5.4.4, v5.4.5, v5.4.6, v5.4.7, v5.4.8, v5.4.9].
    - symfony/swiftmailer-bundle v2.6.7 requires swiftmailer/swiftmailer ~4.2|~5.0 -> satisfiable by swiftmailer/swiftmailer[v5.4.12, v4.2.0, v4.2.1, v4.2.2, v4.3.0, v4.3.1, v5.0.0, v5.0.1, v5.0.2, v5.0.3, v5.1.0, v5.2.0, v5.2.1, v
5.2.2, v5.3.0, v5.3.1, v5.4.0, v5.4.1, v5.4.10, v5.4.11, v5.4.2, v5.4.3, v5.4.4, v5.4.5, v5.4.6, v5.4.7, v5.4.8, v5.4.9].
    - symfony/swiftmailer-bundle v2.6.7 requires swiftmailer/swiftmailer ~4.2|~5.0 -> satisfiable by swiftmailer/swiftmailer[v5.4.12, v4.2.0, v4.2.1, v4.2.2, v4.3.0, v4.3.1, v5.0.0, v5.0.1, v5.0.2, v5.0.3, v5.1.0, v5.2.0, v5.2.1, v
5.2.2, v5.3.0, v5.3.1, v5.4.0, v5.4.1, v5.4.10, v5.4.11, v5.4.2, v5.4.3, v5.4.4, v5.4.5, v5.4.6, v5.4.7, v5.4.8, v5.4.9].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.0.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.0.1].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.0.2].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.0.3].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.1.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.2.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.2.1].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.2.2].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.3.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.3.1].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.4.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.4.1].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.4.2].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.4.3].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v5.4.12].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v4.2.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v4.2.1].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v4.2.2].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v4.3.0].
    - Can only install one of: swiftmailer/swiftmailer[v6.0.0, v4.3.1].
    - cspoo/swiftmailer-mailgun-bundle 1.2.0 requires swiftmailer/swiftmailer ^6.0 -> satisfiable by swiftmailer/swiftmailer[v6.0.0, v6.0.1, v6.0.2, v6.1.0, v6.1.1, v6.1.2, v6.1.3, v6.2.0, v6.2.1, v6.2.2, v6.2.3].
    - Conclusion: don't install swiftmailer/swiftmailer v6.0.1
    - Installation request for symfony/swiftmailer-bundle (locked at v2.6.7, required as ~2.3) -> satisfiable by symfony/swiftmailer-bundle[v2.6.7].


Installation failed, reverting ./composer.json to its original content.

If I try to update symfony/swiftmailer-bundle, here is what I get. I'm lost.

$ composer require  symfony/swiftmailer-bundle --update-with-dependencies
Using version ^3.4 for symfony/swiftmailer-bundle
./composer.json has been updated
Dependency "symfony/symfony" is also a root requirement, but is not explicitly whitelisted. Ignoring.
Dependency "symfony/symfony" is also a root requirement, but is not explicitly whitelisted. Ignoring.
Dependency "symfony/symfony" is also a root requirement, but is not explicitly whitelisted. Ignoring.
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for symfony/swiftmailer-bundle ^3.4 -> satisfiable by symfony/swiftmailer-bundle[v3.4.0].
    - Conclusion: remove symfony/symfony v3.4.36
    - Conclusion: don't install symfony/symfony v3.4.36
    - symfony/swiftmailer-bundle v3.4.0 requires symfony/http-kernel ^4.3.8|^5.0 -> satisfiable by symfony/http-kernel[v4.3.8, v4.3.9, v4.4.0, v4.4.1, v5.0.0, v5.0.1].
    - don't install symfony/http-kernel v4.3.8|don't install symfony/symfony v3.4.36
    - don't install symfony/http-kernel v4.3.9|don't install symfony/symfony v3.4.36
    - don't install symfony/http-kernel v4.4.0|don't install symfony/symfony v3.4.36
    - don't install symfony/http-kernel v4.4.1|don't install symfony/symfony v3.4.36
    - don't install symfony/http-kernel v5.0.0|don't install symfony/symfony v3.4.36
    - don't install symfony/http-kernel v5.0.1|don't install symfony/symfony v3.4.36
    - Installation request for symfony/symfony (locked at v3.4.36, required as 3.4.*) -> satisfiable by symfony/symfony[v3.4.36].


Installation failed, reverting ./composer.json to its original content.