Update dependency webpack-dev-server to v3.9.0

Author: renovate-bot

package-lock.json

@@ -40,6 +40,6 @@
     "vue-template-compiler": "2.6.10",
     "webpack": "4.41.2",
     "webpack-cli": "3.3.9",
-    "webpack-dev-server": "3.8.2"
+    "webpack-dev-server": "3.9.0"
   }
 }