chore(package): update webpack to version 4.39.0

Closes #73

Author: greenkeeper[bot]

package.json

@@ -46,7 +46,7 @@
     "mocha": "^3.2.0",
     "style-loader": "^0.18.0",
     "url-loader": "^0.5.7",
-    "webpack": "^3.1.0",
+    "webpack": "^4.39.0",
     "webpack-dev-server": "^2.3.0"
   },
   "dependencies": {