Downgrade version of webpack-dev-server

The current dependencies caused an error while executing the start script in node v8.9.0. THe issue has been fixed by downgrading the version of webpack-dev-server from 3.0.0 to 2.9.1

package.json

@@ -96,7 +96,7 @@
     "webpack": "^3.6.0",
     "webpack-bundle-analyzer": "^2.9.0",
     "webpack-cli": "^3.1.2",
-    "webpack-dev-server": "^3.0.0",
+    "webpack-dev-server": "^2.9.1",
     "webpack-merge": "^4.1.0"
   },
   "engines": {