Add symfony 2.7 env for travis testing

Author: index0h

.travis.yml

@@ -16,6 +16,7 @@ env:
     - SYMFONY_VERSION="2.4.*"
     - SYMFONY_VERSION="2.5.*"
     - SYMFONY_VERSION="2.6.*"
+    - SYMFONY_VERSION="2.7.*"
 
 before_script:
     - composer self-update