chore(deps): update dependency webpack-dev-server to v3.9.0 (#460)

Author: renovate[bot]

package-lock.json

@@ -84,7 +84,7 @@
     "webpack": "4.41.2",
     "vuepress": "1.2.0",
     "webpack-cli": "3.3.9",
-    "webpack-dev-server": "3.8.2",
+    "webpack-dev-server": "3.9.0",
     "webpack-merge": "4.2.2"
   },
   "browserslist": [