Update rubocop requirement from ~> 1.14.0 to ~> 1.15.0

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@v1.14.0...v1.15.0)

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

Author: dependabot[bot]

Gemfile

@@ -14,5 +14,5 @@ group :development, :test do
 end
 
 group :development do
-  gem 'rubocop', '~> 1.14.0', require: false
+  gem 'rubocop', '~> 1.15.0', require: false
 end