[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](rack/rack@2.0.5...2.0.6)

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

Gemfile.lock

@@ -89,7 +89,7 @@ GEM
     nio4r (2.3.1)
     nokogiri (1.8.5)
       mini_portile2 (~> 2.3.0)
-    rack (2.0.5)
+    rack (2.0.6)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails (5.2.1)