chore(deps): update dependency webpack to v4

Author: renovate-bot

package.json

@@ -47,7 +47,7 @@
     "rimraf": "^2.6.2",
     "source-map-loader": "^0.2.1",
     "url-loader": "^0.6.2",
-    "webpack": "^3.0.0",
+    "webpack": "^4.0.0",
     "webpack-dev-server": "^3.0.0",
     "webpack-node-externals": "^1.6.0"
   },