Update voxpupuli-rubocop requirement from ~> 2.7.0 to ~> 2.8.0

Updates the requirements on [voxpupuli-rubocop](https://github.com/voxpupuli/voxpupuli-rubocop) to permit the latest version.
- [Changelog](https://github.com/voxpupuli/voxpupuli-rubocop/blob/master/CHANGELOG.md)
- [Commits](voxpupuli/voxpupuli-rubocop@v2.7.0...v2.8.0)

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

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

beaker-google.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec', '~> 3.0'
   s.add_development_dependency 'rspec-its', '~> 1.3'
   s.add_development_dependency 'simplecov', '~> 0.22.0'
-  s.add_development_dependency 'voxpupuli-rubocop', '~> 2.7.0'
+  s.add_development_dependency 'voxpupuli-rubocop', '~> 2.8.0'
 
   # Documentation dependencies
   s.add_development_dependency 'thin', '~> 1.8', '>= 1.8.2'