Update rouge requirement from 4.1.3 to 4.2.0 (#31)

Updates the requirements on [rouge](https://github.com/rouge-ruby/rouge) to permit the latest version.
- [Release notes](https://github.com/rouge-ruby/rouge/releases)
- [Changelog](https://github.com/rouge-ruby/rouge/blob/master/CHANGELOG.md)
- [Commits](rouge-ruby/rouge@v4.1.3...v4.2.0)

---
updated-dependencies:
- dependency-name: rouge
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile

@@ -17,4 +17,4 @@ gem 'epubcheck-ruby', '5.1.0.0'
 gem 'html-proofer', '5.0.8'
 gem 'kindlegen', '3.1.1'
 
-gem 'rouge', '4.1.3'
+gem 'rouge', '4.2.0'