Oracle stopped supporting the NetBeans Ruby plugin, but the development has been taken over by the community. In fact, there are now more people working on the plugin than back when Oracle did still support it, including three of the lead developers of JRuby, one of the original developers of the NetBeans Ruby plugin and one former NetBeans core developer, all of whom would probably not be working on it if it was still controlled by Oracle.
They are still using the NetBeans source control repository, the NetBeans project infrastructure, the NetBeans build server, the NetBeans plugin server, … in order to keep the transition as seamless as possible.
Pretty much the only difference is that you can no longer select the plugin directly in the installer, you have to install NetBeans first and then install it via the plugin menu, just like every other plugin, like the Scala or Clojure plugins, for example.
Just download and install the All-In-One Edition, deactivate all options except Base IDE during installation and install the Ruby plugin afterwards.
The plugin center for Ruby from the Continuous Integration server is http://Deadlock.NetBeans.Org/hudson/job/ruby/lastSuccessfulBuild/artifact/build/updates/updates.xml