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
yeah, this avoids doing 2 builds, one for the feature branch and one for the PR opened from it. I only allow builds on master and on branches looking like ``1.0`` (so that we are sure we have build for them if we create a maintenance branch for a release one day)