Update pygments.rb requirement from ~> 2.0.0 to ~> 2.1.0 (#389)
Update pygments.rb requirement from ~> 2.0.0 to ~> 2.1.0 (#389)

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](https://github.com/pygments/pygments.rb/compare/v2.0.0...v2.1.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
asciidoctor/asciidoctor-epub3