Fixed branch alias (#667)

Author: GrahamCampbell

composer.json

@@ -44,7 +44,7 @@
     "prefer-stable": true,
     "extra": {
         "branch-alias": {
-            "dev-master": "2.6.x-dev"
+            "dev-master": "2.7.x-dev"
         }
     }
 }