Update rubocop requirement from 1.38.0 to 1.39.0

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@v1.38.0...v1.39.0)

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

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

Gemfile

@@ -44,7 +44,7 @@ end
 group :test, :development do
   gem 'pry-byebug'
   gem 'redcarpet'
-  gem 'rubocop', '1.38.0'
+  gem 'rubocop', '1.39.0'
 end
 
 gemspec