Update rubocop-rspec requirement from ~> 2.14.1 to ~> 2.15.0

Updates the requirements on [rubocop-rspec](https://github.com/rubocop/rubocop-rspec) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop-rspec/releases)
- [Changelog](https://github.com/rubocop/rubocop-rspec/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-rspec@v2.14.1...v2.15.0)

---
updated-dependencies:
- dependency-name: rubocop-rspec
  dependency-type: direct:development
...

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

colorls.gemspec

@@ -78,7 +78,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rubocop', '~> 1.37.0'
   spec.add_development_dependency 'rubocop-performance', '~> 1.15.0'
   spec.add_development_dependency 'rubocop-rake', '~> 0.5'
-  spec.add_development_dependency 'rubocop-rspec', '~> 2.14.1'
+  spec.add_development_dependency 'rubocop-rspec', '~> 2.15.0'
   spec.add_development_dependency 'rubygems-tasks', '~> 0'
   spec.add_development_dependency 'simplecov', '~> 0.21.2'
   spec.metadata['rubygems_mfa_required'] = 'true'