Merge pull request #479 from ruby/dependabot/bundler/minitest-5.18.0

Update minitest requirement from 5.17.0 to 5.18.0

Author: matzbot

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