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
done in https://github.com/FriendsOfSymfony/FOSHttpCacheBundle/commit/15c79bccc2046a98587035f46fef047a7b936823