Bump rack from 2.0.7 to 2.0.8

Bumps [rack](https://github.com/rack/rack) from 2.0.7 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@2.0.7...2.0.8)

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

Author: dependabot[bot]

Gemfile.lock

@@ -145,7 +145,7 @@ GEM
     pry (0.12.2)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
-    rack (2.0.7)
+    rack (2.0.8)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails (5.2.4)