Update dependency webpack-dev-server to v3.9.0

Author: renovate-bot

package.json

@@ -48,7 +48,7 @@
     "vue-template-compiler": "2.6.10",
     "webpack": "3.12.0",
     "webpack-bundle-analyzer": "3.6.0",
-    "webpack-dev-server": "3.1.11",
+    "webpack-dev-server": "3.9.0",
     "webpack-merge": "4.2.2"
   },
   "engines": {