[build] Update Asciidoctor's JRuby (#1479)
[build] Update Asciidoctor's JRuby (#1479)

To build on JDK 9+, we need to update JRuby to 9.2.1.0 or higher.

This branch builds fine on Bamboo:
https://build.spring.io/browse/REACTOR-RNEXT288-3
reactor/reactor-core