Update Asciidoctor's JRuby

[bsideup]

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

[smaldini]

I guess that's something to squash 💃

[codecov-io]

Codecov Report

Merging #1479 into master will increase coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1479      +/-   ##
============================================
+ Coverage     84.19%   84.25%   +0.06%     
  Complexity     3911     3911              
============================================
  Files           359      359              
  Lines         29760    29760              
  Branches       5527     5527              
============================================
+ Hits          25056    25074      +18     
+ Misses         3078     3070       -8     
+ Partials       1626     1616      -10

Impacted Files	Coverage Δ	Complexity Δ
...or-core/src/main/java/reactor/core/Exceptions.java	90.51% <0%> (-1.73%)	49% <0%> (-1%)
...ava/reactor/core/publisher/FluxWindowBoundary.java	80.45% <0%> (-1.73%)	2% <0%> (ø)
...rc/main/java/reactor/core/publisher/Operators.java	79.94% <0%> (+0.14%)	120% <0%> (ø)	⬇️
...ain/java/reactor/core/publisher/FluxConcatMap.java	90.42% <0%> (+0.28%)	7% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxExpand.java	95.06% <0%> (+0.44%)	3% <0%> (ø)	⬇️
...ava/reactor/core/publisher/EventLoopProcessor.java	80.82% <0%> (+0.51%)	53% <0%> (+1%)	⬆️
...c/main/java/reactor/core/publisher/RingBuffer.java	71.51% <0%> (+0.94%)	24% <0%> (ø)	⬇️
...eactor/core/publisher/ParallelMergeSequential.java	81.34% <0%> (+1.03%)	7% <0%> (ø)	⬇️
...c/main/java/reactor/core/publisher/FluxCreate.java	86.88% <0%> (+1.48%)	8% <0%> (ø)	⬇️
.../java/reactor/core/scheduler/ElasticScheduler.java	85.12% <0%> (+1.65%)	26% <0%> (ø)	⬇️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 518b121...d35e6b7. Read the comment docs.

[simonbasle]

@bsideup for future reference AsciidoctorJ 1.6.0 (newly released) pulls in JRuby 9.2.5.0. When the gradle plugin pulls 1.6.0 we can remove this explicit dependency if needed.