Revert webpack-dev-server to 2.11.1 (#3579)

Author: michael-yx-wu

package.json

@@ -56,7 +56,7 @@
     "tslint": "5.18.0",
     "typescript": "~2.7",
     "webpack": "2.6.1",
-    "webpack-dev-server": "3.1.11",
+    "webpack-dev-server": "2.11.1",
     "webpack-merge": "^4.1.0"
   },
   "dependencies": {