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

Author: nicolas-grekas

Diff for: composer.json

@@ -32,5 +32,7 @@
             "/Tests/"
         ]
     },
-    "version": "3.4.x-dev"
+    "extra": {
+        "branch-version": "3.4-dev"
+    }
 }