Update asciidoctor to 1.5.6.2 and asciidoctor-plantuml to 0.0.8

asciidoctor

Update asciidoctor from 1.5.3 (Oct 2015) to 1.5.6.2 (Mar 2018) to fix many bugs, to improve performance (~ 25%), and to add enhancements.

• Release notes
• Changelog
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asciidoctor-plantuml

Update asciidoctor-plantuml from 0.0.7 (Jan 2017) to 0.0.8 (Feb 2017) to add postprocessor to convert listing to image blocks.

• Diff