Improve Travis tests

[sescandell]

We should improve Travis running tests:

• Test on Symfony 2.7
• Test on Symfony 2.8
• Test on Smfony 3.0

Improve tests globally

[ioleo]

Note that Symfony 3.0 requires "php": ">=5.5.9" (prior versions required "php": ">=5.3.9")