Bump rack from 2.0.1 to 2.0.8 in /slate

Bumps [rack](https://github.com/rack/rack) from 2.0.1 to 2.0.8.
- [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/2.0.1...2.0.8)

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

slate/Gemfile.lock

@@ -96,7 +96,7 @@ GEM
       activesupport (>= 3.1)
     parallel (1.11.1)
     public_suffix (2.0.5)
-    rack (2.0.1)
+    rack (2.0.8)
     rack-livereload (0.3.16)
       rack
     rb-fsevent (0.9.8)