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
looking at https://travis-ci.org/FriendsOfSymfony/FOSHttpCacheBundle this seems to work atm. so we could remove the allow_failure so we notice when we break it. (and the performance gain is pretty impressive btw)