Use unicorn

I was getting annoyed by webrick's double logging (it seems to send duplicate lines of output to stdout), so this should fix that by allowing us to run `foreman start` to get a unicorn server and we'll also be running unicorn on heroku.

For docs, see:

https://github.com/heroku/rails_12factor/issues/10#issuecomment-29522976
https://devcenter.heroku.com/articles/getting-started-with-rails4#webserver

before running `foreman start` locally, run:

    echo "RACK_ENV=development" >> .env
    echo "PORT=3000" >> .env

and make sure to test with RACK_ENV=production before deploying, since that's what heroku uses.