Update rack-test requirement from ~> 1.1 to ~> 2.1

Updates the requirements on [rack-test](https://github.com/rack/rack-test) to permit the latest version.
- [Release notes](https://github.com/rack/rack-test/releases)
- [Changelog](https://github.com/rack/rack-test/blob/main/History.md)
- [Commits](rack/rack-test@v1.1.0...v2.1.0)

---
updated-dependencies:
- dependency-name: rack-test
  dependency-type: direct:production
...

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

Gemfile

@@ -6,7 +6,7 @@ gemspec
 gem "aruba", ">= 0.14"
 gem "citrus", "~> 3.0"
 gem "octokit", "<= 4.22" # 4.22 secretly requires faraday >= 1.0
-gem "rack-test", "~> 1.1"
+gem "rack-test", "~> 2.1"
 gem "rake", "~> 12.3"
 gem "redis", "~> 3.3"
 gem "rspec", "~> 3.3"