Update joints.rb

I've enjoyed converting these sketches to run on ruby-processing, and it alerted me to the idea of ditching the included jruby-complete ( as these sketches will only run properly with an external jruby). However, another more important group of sketches (see topics/shaders) will not run properly with an external jruby (at least on my setup). So for now I'm inclined to stick with including jruby-complete.
@jashkenas and @alexdean.