fix(package): update workbox-webpack-plugin to version 4.1.1

Closes #201

Author: greenkeeper[bot]

package.json

@@ -85,7 +85,7 @@
     "webpack-dev-server": "3.2.1",
     "webpack-merge": "4.2.1",
     "which": "1.3.1",
-    "workbox-webpack-plugin": "3.6.3",
+    "workbox-webpack-plugin": "4.1.1",
     "yargs": "13.2.2"
   },
   "devDependencies": {