A native extension was added last year that prevents the gem from being installable on JRuby. We need to either replace that with FFI/fiddle code, pure-Ruby, or a JRuby version.
This relates to jruby/jruby#8649. We do not install the resolv gem at build time for JRuby, but some dependency requires it and can't install because of this extension.
Short term, removing the extension and pushing a -java platform gem would fix all issues installing on JRuby, but we need a long-term solution.