[Travis] Add env for SF 2.7 and 2.8

Author: francoispluchino

.travis.yml

@@ -21,6 +21,8 @@ env:
     - SYMFONY_VERSION=2.4.*
     - SYMFONY_VERSION=2.5.*
     - SYMFONY_VERSION=2.6.*
+    - SYMFONY_VERSION=2.7.*
+    - SYMFONY_VERSION=2.8.*
 
 install:
     - composer self-update