Update pygments.rb requirement from 2.2.0 to 2.3.1

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@v2.2.0...v2.3.1)

---
updated-dependencies:
- dependency-name: pygments.rb
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Gemfile

@@ -10,7 +10,7 @@ gem 'asciidoctor-epub3', '1.5.1'
 gem 'asciidoctor-pdf', '1.6.1'
 
 gem 'coderay', '1.1.3'
-gem 'pygments.rb', '2.2.0'
+gem 'pygments.rb', '2.3.1'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '4.2.6.0'
 gem 'html-proofer', '3.19.3'