fixup dev-master alias

Author: bburnichon

composer.json

@@ -52,7 +52,7 @@
     "bin": ["bin/validate-json"],
     "extra": {
         "branch-alias": {
-            "dev-master": "1.4.x-dev"
+            "dev-master": "1.6.x-dev"
         }
     }
 }