Prevent a warning when installing the lib

[dunglas]

Q	A
Bug fix?	yes
New feature?	yes
BC breaks?	yes
Related tickets	n/a
License	MIT

Prevents the following Flex warning when installing the library:

 Adding phpunit/phpunit as a dependency is discouraged in favor of Symfony's PHPUnit Bridge. 
                                                                                             
  * Instead:
    1. Remove it now: composer remove --dev phpunit/phpunit
    2. Use Symfony's bridge: composer require --dev phpunit

[lchrusciel]

Thanks a lot Kevin! I want to take a look and do some refactor anyway around this, so I can pick it up

[lchrusciel]

I know, it took me a while, but I finally found some time to do something about it. Unfortunately, I wasn't able to force phpunit bridge to see ApiTestCase, because it is using its own composer autoload definition :(

[lchrusciel]

Ref. https://travis-ci.org/lchrusciel/ApiTestCase/jobs/564395812