chore(deps): update dependency workbox-build to v6

Author: renovate-bot

package.json

@@ -62,7 +62,7 @@
     "standard-version": "5.0.2",
     "tslint": "5.16.0",
     "tslint-react": "4.0.0",
-    "workbox-build": "4.3.1"
+    "workbox-build": "6.1.5"
   },
   "repository": {
     "type": "git",