[Security] Bump rack from 2.0.8 to 2.2.2

Bumps [rack](https://github.com/rack/rack) from 2.0.8 to 2.2.2. **This update includes a security fix.**
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.8...v2.2.2)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -3,7 +3,7 @@ GEM
   specs:
     mustermann (1.1.0)
       ruby2_keywords (~> 0.0.1)
-    rack (2.0.8)
+    rack (2.2.2)
     rack-protection (2.0.8.1)
       rack
     redis (4.1.3)