Simplified the Travis configuration

The tests against specific Symfony versions are now running only for a single PHP version to limit the number of jobs in the build matrix. They are also installing the full Symfony repo to be sure that all components are actually at the specified version without the need to require each of them explicitly.
Fixed the retrieval of the validation MetadataFactory

The service is private so getting it from the container get() method is invalid and it does not work anymore in Symfony 2.5 because the service gets inlined.
Merge pull request #409 from stof/patch-2

Simplified the Travis configuration