Update AsciidoctorJ, updating parser for syntax highlighting

• Upgrade to AsciidoctorJ 1.6.1 and JRuby 9.2.5.0 (it's still backed by Asciidoctor 1.5.8)
• Upgrade to asciidoctor diagram 1.5.11
• Updated parser for old style multiline headings to be more specific (#233)
• Added description for old style heading inspection (#233)