Thanks to recent commits from Wayne, you can now pass a build flag when install jruby with rvm and it will build jruby instead of downloading a prebuilt copy. Make sure you rvm get head && rvm reload first. Then we can install jruby with 1.9 mode as the default:
rvm install jruby -C -Djruby.default.ruby.version=1.9And if you want to use jruby-head:
rvm install jruby-head -C -Djruby.default.ruby.version=1.9