Update rubocop-minitest requirement from = 0.14.0 to = 0.15.0

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@v0.14.0...v0.15.0)

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

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

lazylead.gemspec

@@ -91,7 +91,7 @@ tasks instead of solving technical problems."
   s.add_development_dependency "random-port", "0.5.1"
   s.add_development_dependency "rdoc", "6.3.2"
   s.add_development_dependency "rubocop", "1.18.4"
-  s.add_development_dependency "rubocop-minitest", "0.14.0"
+  s.add_development_dependency "rubocop-minitest", "0.15.0"
   s.add_development_dependency "rubocop-performance", "1.11.4"
   s.add_development_dependency "rubocop-rake", "0.6.0"
   s.add_development_dependency "rubocop-rspec", "2.4.0"