Native extension prevents install on JRuby

[headius]

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 https://github.com/jruby/jruby/issues/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.

[headius]

This is moderately critical because the release of resolv 0.6.0 appears to have broken all JRuby builds prior to JRuby 9.4.10.0. Builds from that point on skip native extensions during default gem install, so they do not have the issue.

There is a separate bug on the JRuby side: we are not ready to use the resolv gem, but it is being installed at build time. I can't fix that for old versions, though, so we really need a stub gem for resolv ASAP so those old builds will work again.

cc @hsbt @nobu