chore(package): update webpack-dev-server to version 3.9.0

Closes #334

Author: greenkeeper[bot]

package.json

@@ -85,7 +85,7 @@
     "style-loader": "^0.21.0",
     "url-loader": "^1.0.0",
     "webpack": "^3.0.0",
-    "webpack-dev-server": "^2.4.4"
+    "webpack-dev-server": "^3.9.0"
   },
   "dependencies": {
     "preact": "^8.1.0",