Use puma

First of all, since I can't get therubyracer to work on my machine, I cannot actually bundle install, so someone else will have to add a commit to this that updates the lock.

Secondly, no matter what we do, we shouldn't have both thin and unicorn in our gemfile, that's stupid.

Third: puma.

Given https://blog.heroku.com/archives/2013/2/16/routing_performance_update/ and some recent research I have done, thin is a bad choice. We're not using resources as well as we could be. Second, unicorn is a bad choice: we want to support development on Windows.

That seems to leave puma. Puma is windows-friendly, mega fast, and has a good concurrency track record.