Revert to Ruby 2.0

Ruby 2.1 on Heroku seems to have some GC issues and our app was being inundated with R14 memory errors. More info and discussion here: https://discussion.heroku.com/t/tuning-rgengc-2-1-on-heroku/359/11. We'll need to go back to 2.1 and fine tune the GC variables soon, but this should be a temporary fix until the demos are done.