[Travis] Change Symfony 2.3 for ~2.6@beta testing

We aim to support Symfony >=2.5,<=2.7.0 in eZ Publish 5.4, so this introduces some early testing to make sure we are ready for that.
[Travis] Update hhvm rules for symfony beta testing
Merge pull request #1068 from ezsystems/symfony-beta-testing

[Travis] Swap Symfony 2.3 for ~2.6@beta testing