chore(package): update webpack to version 4.20.0

Closes #191

Author: greenkeeper[bot]

package.json

@@ -142,7 +142,7 @@
     "tslint": "^4.5.0",
     "typedoc": "^0.5.7",
     "typescript": "^2.2.1",
-    "webpack": "^2.2.1",
+    "webpack": "^4.20.0",
     "webpack-merge": "^3.0.0"
   },
   "babel": {