Update rubocop-minitest requirement from ~> 0.28.0 to ~> 0.30.0

Updates the requirements on [rubocop-minitest](https://github.com/rubocop/rubocop-minitest) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop-minitest/releases)
- [Changelog](https://github.com/rubocop/rubocop-minitest/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-minitest@v0.28.0...v0.30.0)

---
updated-dependencies:
- dependency-name: rubocop-minitest
  dependency-type: direct:production
...

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

Gemfile

@@ -5,7 +5,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'rubocop', '~> 1.41'
-gem 'rubocop-minitest', '~> 0.28.0'
+gem 'rubocop-minitest', '~> 0.30.0'
 gem 'rubocop-performance', '~> 1.15'
 gem 'rubocop-rails', '~> 2.17'
 gem 'rubocop-rake', '~> 0.6.0'