Merge branch '5.1' into 5.x

* 5.1:
  fix merge
  [appveyor] fix checking for the .x branch
  Remove "branch-alias", populate "version"

Author: nicolas-grekas

composer.json

@@ -40,9 +40,5 @@
         ]
     },
     "minimum-stability": "dev",
-    "extra": {
-        "branch-alias": {
-            "dev-master": "5.2-dev"
-        }
-    }
+    "version": "5.2-dev"
 }