Bump rack from 2.2.6.3 to 2.2.6.4 in /resque-web (#928)

Bumps [rack](https://github.com/rack/rack) from 2.2.6.3 to 2.2.6.4.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@v2.2.6.3...v2.2.6.4)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: indirect
...

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resque-web/Gemfile.lock

@@ -12,7 +12,7 @@ GEM
     mustermann (2.0.2)
       ruby2_keywords (~> 0.0.1)
     raabro (1.1.6)
-    rack (2.2.6.3)
+    rack (2.2.6.4)
     rack-protection (2.2.3)
       rack
     rake (12.3.3)