composer.json: adding "prefer-stable": true let composer choose stable packages

With `"prefer-stable": true` in composer.json Composer will use stable versions rather than commits like `2.2.x-dev-...`.

Author: k00ni

composer.json

@@ -39,5 +39,6 @@
             "application/controllers/"
         ]
     },
-    "minimum-stability": "dev"
+    "minimum-stability": "dev",
+    "prefer-stable": true
 }