Bump rack from 1.6.4 to 1.6.13

Bumps [rack](https://github.com/rack/rack) from 1.6.4 to 1.6.13.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.6.4...1.6.13)

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

Author: dependabot[bot]

Gemfile.lock

@@ -11,7 +11,7 @@ GEM
     mini_portile2 (2.4.0)
     nokogiri (1.10.10)
       mini_portile2 (~> 2.4.0)
-    rack (1.6.4)
+    rack (1.6.13)
     rack-protection (1.5.3)
       rack
     sinatra (1.4.6)