Update rubocop requirement from ~> 1.28.2 to ~> 1.50.2

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@v1.28.2...v1.50.2)

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

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

asciidoctor-epub3.gemspec

@@ -37,7 +37,7 @@ An extension for Asciidoctor that converts AsciiDoc documents to EPUB3 and KF8/M
   s.add_development_dependency 'rake', '~> 13.1.0'
   s.add_development_dependency 'rouge', '~> 3.0'
   s.add_development_dependency 'rspec', '~> 3.12.0'
-  s.add_development_dependency 'rubocop', '~> 1.28.2'
+  s.add_development_dependency 'rubocop', '~> 1.50.2'
   s.add_development_dependency 'rubocop-rake', '~> 0.6.0'
   s.add_development_dependency 'rubocop-rspec', '~> 2.10.0'