[Security] Bump rack from 2.0.5 to 2.0.6

Bumps [rack](https://github.com/rack/rack) from 2.0.5 to 2.0.6. **This update includes security fixes.**
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rack/rack/compare/2.0.5...2.0.6)

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

Gemfile.lock

@@ -407,7 +407,7 @@ GEM
     public_suffix (3.0.2)
     rabl (0.13.1)
       activesupport (>= 2.3.14)
-    rack (2.0.5)
+    rack (2.0.6)
     rack-accept (0.4.5)
       rack (>= 0.4)
     rack-attack (5.2.0)