Drop support for composer v1

[swissspidy]

• Packagist is dropping support for composer v1
• composer v1 uses an outdated version of symfony/process, causing installs to fail on PHP 5.6

composer/composer 2.2 still supports PHP 5.3+ so that's no issue.

In the future we can bump the minimum requirement to v2.8+ (requires PHP 7.2+)

Fixes #194

See wp-cli/wp-cli#6018

[swissspidy]

Or actually, it might not fix wp-cli/wp-cli#6018 after all. But still something we'd need to do.