Update rubocop requirement from ~> 1.50.0 to ~> 1.51.0

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@v1.50.0...v1.51.0)

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

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

Gemfile

@@ -7,7 +7,7 @@ gemspec
 group :development do
   gem 'rake', '>= 10.0.0'
   gem 'rspec', '~> 3.12.0'
-  gem 'rubocop', '~> 1.50.0'
+  gem 'rubocop', '~> 1.51.0'
   gem 'rubocop-performance', '~> 1.17.1'
   gem 'rubocop-rake', '~> 0.6.0'
   gem 'rubocop-rspec', '~> 2.22.0'