chore(package): update webpack-dev-server to version 3.1.12

Closes #12

Author: greenkeeper[bot]

package.json

@@ -44,7 +44,7 @@
     "typescript": "^2.4.2",
     "webpack": "^3.3.0",
     "webpack-dev-middleware": "^1.5.1",
-    "webpack-dev-server": "^2.6.0",
+    "webpack-dev-server": "^3.1.12",
     "webpack-notifier": "^1.3.0"
   }
 }