Update rubocop requirement from 1.59.0 to 1.60.2

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@v1.59.0...v1.60.2)

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

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile

@@ -5,7 +5,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'coveralls', require: false
-gem 'rubocop', '1.59.0', require: false
+gem 'rubocop', '1.60.2', require: false
 gem 'rubocop-performance', '1.20.1', require: false
 gem 'rubocop-rake', '0.6.0', require: false
 gem 'rubocop-rspec', '2.25.0', require: false