[Security] Bump rack from 2.0.7 to 2.2.1

Bumps [rack](https://github.com/rack/rack) from 2.0.7 to 2.2.1. **This update includes security fixes.**
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.7...v2.2.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Gemfile.lock

@@ -175,7 +175,7 @@ GEM
       nio4r (~> 2.0)
     puma (4.1.1-java)
       nio4r (~> 2.0)
-    rack (2.0.7)
+    rack (2.2.1)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails (5.2.3)