Fix EZP-22597: Use composition on extended FragmentRenderers
EZP-22597: Fix tests
EZP-22597: Added tests
EZP-22597: Updated .travis.yml to test against Symfony 2.3 and latest
Merge pull request #793 from ezsystems/implement_EZP-22597_fragmentRenderersComposition

Fix EZP-22597: Use composition on extended FragmentRenderers