chore(package): update webpack-dev-server to version 2.9.1 (#165)

package.json

@@ -95,7 +95,7 @@
     "typescript": "^2.5.3",
     "uglifyjs-webpack-plugin": "^0.4.3",
     "webpack": "^3.5.5",
-    "webpack-dev-server": "^2.7.1"
+    "webpack-dev-server": "^2.9.1"
   },
   "config": {
     "commitizen": {