chore(package): update webpack to version 4.9.0

Closes #60

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "ts-loader": "^2.3.3",
     "typedoc": "^0.8.0",
     "typescript": "^2.4.2",
-    "webpack": "^3.5.5",
+    "webpack": "^4.9.0",
     "webpack-shell-plugin": "^0.5.0"
   },
   "peerDependencies": {