chore(package): update webpack to version 4.1.0

Closes #11

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "koa": "^2.3.0",
     "react": "^15.6.1",
     "react-dom": "^15.6.1",
-    "webpack": "^3.5.4"
+    "webpack": "^4.1.0"
   },
   "dependencies": {
     "require-from-string": "^2.0.0",