Update rubocop requirement from 1.7.0 to 1.8.0

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@v1.7.0...v1.8.0)

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

Gemfile

@@ -5,7 +5,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem 'coveralls', require: false
-gem 'rubocop', '1.7.0', require: false
+gem 'rubocop', '1.8.0', require: false
 gem 'rubocop-performance', require: false
 gem 'rubocop-rake', require: false
 gem 'rubocop-rspec', require: false