Update rubocop requirement from ~> 0.89.1 to ~> 0.90.0 (#21)

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@v0.89.1...v0.90.0)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -7,6 +7,6 @@ gemspec
 
 gem 'minitest', '~> 5.0'
 gem 'rake', '~> 13.0'
-gem 'rubocop', '~> 0.89.1'
+gem 'rubocop', '~> 0.90.0'
 gem 'simplecov', '~> 0.19.0', require: false # CodeClimate not compatible with 0.18+ yet - https://github.com/codeclimate/test-reporter/issues/413
 gem 'simplecov-lcov', '< 0.8'