Bump rack from 2.1.1 to 2.2.3

Bumps [rack](https://github.com/rack/rack) from 2.1.1 to 2.2.3.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.1.1...2.2.3)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -3,7 +3,7 @@ GEM
   specs:
     mustermann (1.1.1)
       ruby2_keywords (~> 0.0.1)
-    rack (2.1.1)
+    rack (2.2.3)
     rack-protection (2.0.8.1)
       rack
     ruby2_keywords (0.0.2)