Remove "version" from composer.json files, use "branch-version" instead

Author: nicolas-grekas

composer.json

@@ -57,5 +57,7 @@
         ]
     },
     "minimum-stability": "dev",
-    "version": "3.4.x-dev"
+    "extra": {
+        "branch-version": "3.4-dev"
+    }
 }