jruby with 1.9 mode as the default with rvm

Thanks to [recent commits](https://github.com/wayneeseguin/rvm/compare/249d08df…2388c1af) from [Wayne](https://github.com/wayneeseguin), 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=1.9

And if you want to use jruby-head:

rvm install jruby-head -C -Djruby.default.ruby.version=1.9