Also set the master branch alias in 2.x branch

Author: acrobat

composer.json

@@ -44,7 +44,8 @@
     "prefer-stable": true,
     "extra": {
         "branch-alias": {
-            "dev-2.x": "2.15.x-dev"
+            "dev-2.x": "2.15.x-dev",
+            "dev-master": "3.0.x-dev"
         }
     }
 }