chore(package): update webpack to version 4.21.0

Author: greenkeeper[bot]

package.json

@@ -110,7 +110,7 @@
     "tslint-config-prettier": "^1.15.0",
     "typescript": "^3.1.1",
     "uglifyjs-webpack-plugin": "^2.0.1",
-    "webpack": "^4.20.2",
+    "webpack": "^4.21.0",
     "webpack-cli": "^3.1.2",
     "webpack-dev-server": "^3.1.9"
   }