Update rainbow requirement from ~> 2.2.2 to ~> 3.1.1

Updates the requirements on [rainbow](https://github.com/sickill/rainbow) to permit the latest version.
- [Changelog](https://github.com/ku1ik/rainbow/blob/master/Changelog.md)
- [Commits](ku1ik/rainbow@v2.2.2...v3.1.1)

---
updated-dependencies:
- dependency-name: rainbow
  dependency-type: direct:development
...

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

Author: dependabot[bot]

sshkit.gemspec

@@ -30,7 +30,7 @@ Gem::Specification.new do |gem|
   gem.add_development_dependency('danger')
   gem.add_development_dependency('minitest', '>= 5.0.0')
   gem.add_development_dependency('minitest-reporters')
-  gem.add_development_dependency('rainbow', '~> 2.2.2')
+  gem.add_development_dependency('rainbow', '~> 3.1.1')
   gem.add_development_dependency('rake')
   gem.add_development_dependency('rubocop', "~> 0.52.0")
   gem.add_development_dependency('mocha')