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>

Author: dependabot[bot]

modulesync.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'rspec'
   spec.add_development_dependency 'simplecov'
-  spec.add_development_dependency 'voxpupuli-rubocop', '~> 2.7.0'
+  spec.add_development_dependency 'voxpupuli-rubocop', '~> 2.8.0'
 
   spec.add_runtime_dependency 'git', '~>1.7'
   spec.add_runtime_dependency 'gitlab', '~>4.0'