Bump rack from 2.0.7 to 2.0.8 (#914)

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>

Gemfile.lock

@@ -215,7 +215,7 @@ GEM
     popper_js (1.14.5)
     public_suffix (3.1.1)
     puma (3.12.2)
-    rack (2.0.7)
+    rack (2.0.8)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails (5.2.3)