Enabled Symfony 2.6 and 2.7 and disabled Symfony 3.x in travis
Disable deprecated errors for symfony 2.7 and higher
Merge pull request #502 from boekkooi/patch-1

[Travis-CI] Enabled Symfony 2.6 and 2.7 and disabled Symfony 3.x
Merge remote-tracking branch 'sonata-project/master'