Add definition for JRuby 9.2.8.0
Add definition for JRuby 9.2.8.0

Closes #1335.

JRuby 9.2.8.0 has been released.
https://www.jruby.org/2019/08/12/jruby-9-2-8-0.html

ryanfb/ruby-build,sstephenson/ruby-build,rbenv/ruby-build,rbenv/ruby-build,OiNutter/node-build,makandra/ruby-build,dark-panda/ruby-build,nodenv/node-build,dark-panda/ruby-build