added missing branch-alias in composer file

Author: fabpot

composer.json

@@ -36,6 +36,9 @@
     },
     "extra": {
         "symfony-app-dir": "app",
-        "symfony-web-dir": "web"
+        "symfony-web-dir": "web",
+        "branch-alias": {
+            "dev-master": "2.1-dev"
+        }
     }
 }