Changed Server to Puma

Updated server to Puma following the instructions here: http://blog.codeship.io/2013/10/16/unleash-the-puma-on-heroku.html

Based on preliminary tests, it's squeezing significantly more performance out of a single dyno than out of our plurality of Thin dynos we ran earlier today.

These are the environment variables I used:
PUMA_MAX_THREADS:           10
PUMA_MIN_THREADS:           10
PUMA_WORKERS:               2
DB_POOL:                    15