Downgrade to 2.11 as webpacker-dev-server not running in higer version rails/webpacker#1303

Author: coderhs

package.json

@@ -21,6 +21,6 @@
     "husky": "^0.13.4",
     "lint-staged": "^3.6.0",
     "prettier": "^1.4.2",
-    "webpack-dev-server": ">=3.1.11"
+    "webpack-dev-server": "^2.11.1"
   }
 }