Update pygments.rb requirement from 2.0.0 to 2.1.0

Updates the requirements on [pygments.rb](https://github.com/pygments/pygments.rb) to permit the latest version.
- [Release notes](https://github.com/pygments/pygments.rb/releases)
- [Changelog](https://github.com/pygments/pygments.rb/blob/master/CHANGELOG.adoc)
- [Commits](pygments/pygments.rb@v2.0.0...v2.1.0)

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

Author: dependabot[bot]

Gemfile

@@ -10,7 +10,7 @@ gem 'asciidoctor-epub3', '1.5.0.alpha.19'
 gem 'asciidoctor-pdf', '1.5.4'
 
 gem 'coderay', '1.1.3'
-gem 'pygments.rb', '2.0.0'
+gem 'pygments.rb', '2.1.0'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '4.2.4.0'
 gem 'html-proofer', '3.18.5'