[WIP] Test lowest versions of vendor packages

* Added tests for the lowest versions allowed by the composer.json
* Updated the version constraints so that they only allow truly supported versions
* Bumped the PHP version up to 5.3.3, as you can never proof that you support 5.3.0 (it is no longer supported by Travis). It didn't make sense too, since Symfony Console is requiring 5.3.3.
* Added tests for PHP 5.6