Bump rack from 1.6.11 to 1.6.13

Bumps [rack](https://github.com/rack/rack) from 1.6.11 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@1.6.11...1.6.13)

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

Author: dependabot[bot]

Gemfile.lock

@@ -44,7 +44,7 @@ GEM
       forwardable-extended (~> 2.6)
     public_suffix (3.0.3)
     puma (3.12.0)
-    rack (1.6.11)
+    rack (1.6.13)
     rack-jekyll (0.5.0)
       jekyll (>= 1.3)
       listen (>= 1.3)