While you can probably package up your code as a JRuby application with JAR files and Java byte code, there are decompilers for that, so you can never presume your source to be 100% secure.
Ideally you'd offer some sort of VM appliance that the customer can install, a system image compatible with VMWare or whatever virtualization system they're using. This helps package up a fairly secure environment, but won't protect against a determined adversary trying to get your source code.
If you're giving out your code to someone in any form, compiled or otherwise, you have to have a certain amount of trust. Even compiled executables are not immune to reverse-engineering.