Bump rack from 2.0.6 to 2.0.8

Bumps [rack](https://github.com/rack/rack) from 2.0.6 to 2.0.8.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.6...2.0.8)

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

Author: dependabot[bot]

Gemfile.lock

@@ -41,7 +41,7 @@ GEM
     minitest (5.11.3)
     nokogiri (1.9.1)
       mini_portile2 (~> 2.4.0)
-    rack (2.0.6)
+    rack (2.0.8)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails-dom-testing (2.0.3)