update the branch alias

Since adding support for the pull request review API is a new feature
the minor version needs to be incremented.

Author: xabbuh

composer.json

@@ -37,7 +37,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "2.0.x-dev"
+            "dev-master": "2.1.x-dev"
         }
     }
 }