Update rubocop requirement from ~> 0.87.0 to ~> 0.89.1

Updates the requirements on [rubocop](https://github.com/rubocop-hq/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop-hq/rubocop/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v0.87.0...v0.89.1)

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

Gemfile

@@ -12,7 +12,7 @@ end
 group :development do
   gem 'rake', '~> 13.0'
   gem 'rspec', '~> 3.9'
-  gem 'rubocop', '~> 0.87.0'
+  gem 'rubocop', '~> 0.89.1'
   gem 'rubocop-performance', '~> 1.7'
   gem 'rubocop-rspec', '~> 1.41'
   gem 'truthy', '~> 1.0'