Update pygments.rb requirement from ~> 2.3.0 to ~> 2.4.0 (#440)

* Update pygments.rb requirement from ~> 2.3.0 to ~> 2.4.0

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@v2.3.0...v2.4.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

* Fix tests

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Marat Radchenko <marat@slonopotamus.org>

asciidoctor-epub3.gemspec

@@ -33,7 +33,7 @@ An extension for Asciidoctor that converts AsciiDoc documents to EPUB3 and KF8/M
   s.add_development_dependency 'asciidoctor-diagram', '>= 1.5.0', '< 3.0.0'
   s.add_development_dependency 'asciimath', '~> 2.0'
   s.add_development_dependency 'coderay', '~> 1.1.0'
-  s.add_development_dependency 'pygments.rb', '~> 2.3.0'
+  s.add_development_dependency 'pygments.rb', '~> 2.4.0'
   s.add_development_dependency 'rake', '~> 13.0.0'
   s.add_development_dependency 'rouge', '~> 3.0'
   s.add_development_dependency 'rspec', '~> 3.12.0'

spec/highlight_spec.rb

@@ -21,7 +21,7 @@
     article = book.item_by_href 'source_highlight.xhtml'
     expect(article).not_to be_nil
     if Asciidoctor::Document.supports_syntax_highlighter?
-      expect(article.content).to include '<span class="tok-k">class</span> <span class="tok-nc">Foo</span>'
+      expect(article.content).to include '<span class="tok-nc">Foo</span>'
       expect(article.content).to include '<link rel="stylesheet" href="./pygments-bw.css"/>'
       expect(book.item_by_href('pygments-bw.css')).not_to be_nil
     else