Bump workerpool from 6.0.0 to 9.1.1 in /benchmark (#78)

Bumps [workerpool](https://github.com/josdejong/workerpool) from 6.0.0 to 9.1.1.
- [Changelog](https://github.com/josdejong/workerpool/blob/master/HISTORY.md)
- [Commits](josdejong/workerpool@v6.0.0...v9.1.1)

---
updated-dependencies:
- dependency-name: workerpool
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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benchmark/package.json

@@ -4,7 +4,7 @@
     "process-pool": "^0.3.5",
     "worker-farm": "^1.7.0",
     "worker-nodes": "^1.6.1",
-    "workerpool": "^6.0.0"
+    "workerpool": "^9.1.1"
   },
   "devDependencies": {
     "bluebird": "^3.7.2",