Bump rack from 1.5.2 to 1.6.13 in /api/ruby/building-a-ci-server (#316)

Bumps [rack](https://github.com/rack/rack) from 1.5.2 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@1.5.2...1.6.13)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

api/ruby/building-a-ci-server/Gemfile.lock

@@ -8,7 +8,7 @@ GEM
     multipart-post (2.0.0)
     octokit (3.0.0)
       sawyer (~> 0.5.3)
-    rack (1.5.2)
+    rack (1.6.13)
     rack-protection (1.5.2)
       rack
     sawyer (0.5.4)