Upgrade to asciidoctor 2.0.20

[robertpanzer]

Thank you for opening a pull request and contributing to AsciidoctorJ!

Please take a bit of time giving some details about your pull request:

Kind of change

• [ ] Bug fix
• [x] New non-breaking feature
• [ ] New breaking feature
• [ ] Documentation update
• [ ] Build improvement

Description

What is the goal of this pull request?

Upgrade to the latest and greatest version of asciidoctor, 2.0.20.

How does it achieve that?

Are there any alternative ways to implement this?

Are there any implications of this pull request? Anything a user must know?

Issue

If this PR fixes an open issue, please add a line of the form:

Fixes #Issue

Release notes

Please add a corresponding entry to the file CHANGELOG.adoc