[Security] 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. **This update includes security fixes.**
- [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-preview[bot] <support@dependabot.com>

Gemfile.lock

@@ -287,7 +287,7 @@ GEM
       nokogiri (~> 1.6)
       rails (>= 4.2.0, < 6.0)
       rdf
-    rack (1.6.11)
+    rack (1.6.13)
     rack-dev-mark (0.7.5)
       rack (>= 1.1)
     rack-test (0.6.3)