JRuby is indeed your answer.
I used to work for a company that did exactly this. We had an on
premise enterprise server we were selling and distributing to clients
written in ruby. Yes, we did WAR it all up too, but that's it what
you're looking for.
JRuby has the ability to *actually compile* your ruby code into
java .class files. This has some clear performance benefits since your
rb files aren't being interpreted at runtime anymore, but it also
gives you some obvious advantages when your distributing your code.
In my opinion it's the only decent way to distribute ruby. There's
loads of documentation on the topic if you look it up.
Also, one other quick word of advice: watch those license agreements
in your dependencies carefully.
Much of the awesome open source code we love and enjoy in the ruby
community has entire different rules when your distributing it vs
running it on a web server. Have your lawyers check it over good. The
good news is though that if JRuby also lets you leverage java
libraries in your ruby code so you can no doubt find what you need.