Update dependency webpack to v4

Author: renovate-bot

package.json

@@ -35,7 +35,7 @@
     "react-test-renderer": "^16.0.0",
     "standard": "^11.0.0",
     "style-loader": "^0.20.0",
-    "webpack": "^3.5.5",
+    "webpack": "^4.0.0",
     "webpack-dev-server": "^2.7.1",
     "x-iterable": "^6.0.1"
   },