To run install Java, JRuby and all the gems in gems.txt.
You can find JRuby 1.4.0 from here.
This example uses geekq's jetty-rackup project.
Start application in jetty container
$ jruby jetty-rackup
Run specs
$ jruby -S rake spec
For testing there is rspec configured with mocha, rack/test and webrat