Update minitest requirement from 5.17.0 to 5.18.0

Updates the requirements on [minitest](https://github.com/seattlerb/minitest) to permit the latest version.
- [Release notes](https://github.com/seattlerb/minitest/releases)
- [Changelog](https://github.com/minitest/minitest/blob/master/History.rdoc)
- [Commits](minitest/minitest@v5.17.0...v5.18.0)

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

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

Author: dependabot[bot]

Gemfile

@@ -4,7 +4,7 @@ gemspec
 
 group :development do
   gem "bundler"
-  gem "minitest", "5.17.0"
+  gem "minitest", "5.18.0"
   gem "coveralls"
   gem "rubocop", "~> 1.12.1"
 end