Improved the Travis configuration

- add testing on HHVM and PHP 5.6
- test the non-latest versions of Symfony only once instead of doing it for all PHP versions, thus reducing the matrix
- build only the master branch and the release branches to avoid extra builds when using the main repo for feature branches
- let composer use individual components and bundles for the normal resolution of Symfony
2.5 is tested by the normal build as Composer will select the latest release