chore(package): update webpack to version 4.20.0

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "mocha": "^5.0.0",
     "npm-run-all": "^4.1.2",
     "text-encoding": "^0.6.0",
-    "webpack": "4.18.1",
+    "webpack": "4.20.0",
     "webpack-cli": "^2.0.13"
   },
   "author": {