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

We aim to support Symfony >=2.5,<=2.7 in eZ Publish 5.4, so this introduces some early testing to make sure we are ready for that.


Related to https://github.com/ezsystems/ezpublish-community/pull/201 where BDD tests uncovered some REST issues with Symfony 2.6-Beta1 (ping @bdunogier / @lolautruche ).