Update rubocop requirement from ~> 0.51 to ~> 1.15

Updates the requirements on [rubocop](https://github.com/rubocop/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v0.51.0...v1.15.0)

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

pushmi_pullyu.gemspec

@@ -42,7 +42,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'pry-byebug', '~> 3.6'
   spec.add_development_dependency 'rake', '~> 13.0'
   spec.add_development_dependency 'rspec', '~> 3.0'
-  spec.add_development_dependency 'rubocop', '~> 0.51'
+  spec.add_development_dependency 'rubocop', '~> 1.15'
   spec.add_development_dependency 'rubocop-rspec', '~> 1.10'
   spec.add_development_dependency 'timecop', '~> 0.8'
   spec.add_development_dependency 'vcr', '~> 5.0'