Switch Symfony 2.6 to stable and test on 2.7 and master

Author: ddeboer

.travis.yml

@@ -26,7 +26,15 @@ matrix:
         - FRAMEWORK_EXTRA_VERSION='~3.0'
     - php: 5.5
       env:
-        - SYMFONY_VERSION='2.6.*@dev'
+        - SYMFONY_VERSION='2.6.*'
+        - FRAMEWORK_EXTRA_VERSION='~3.0'
+    - php: 5.5
+      env:
+        - SYMFONY_VERSION='2.7.*@dev'
+        - FRAMEWORK_EXTRA_VERSION='~3.0'
+    - php: 5.5
+      env:
+        - SYMFONY_VERSION='dev-master'
         - FRAMEWORK_EXTRA_VERSION='~3.0'
 
 before_script: