When listing subdirectories under http://ci.jruby.org/snapshots/ there are no Mac OS X installers. Could you add these?
I see now that the reason probably is that the jruby-dist-* targets are not run on an OS X machine, so the dmg is skipped.
I guess the only way to fix this is to add an OS X Jenkins slave.
Anybody got one handy? I do not.
Closing this since we're not likely to get an OS X slave anytime soon.