Switch web server from Thin to Unicorn

As the Rails application scales to more Heroku dynos, a lack of
concurrency can result in [poor performance](1) and an over-provisioning
of resources. Modifying the app to handle requests more efficiently is
a simple process with immediate benefits.

Unicorn is a [concurrent web server](2) that spawns several processes
within a single dyno without requiring concurrency or threading
awareness in your app.  Applications that migrate to Unicorn often
require fewer dynos and see increased performance.

The [config/unicorn.rb file](3) specifies the number of concurrent web
processes to run on each dyno as well as the proper forking and
termination behavior.

[1] https://blog.heroku.com/archives/2013/2/16/routing_performance_update
[2] https://blog.heroku.com/archives/2013/2/27/unicorn_rails
[3] https://devcenter.heroku.com/articles/rails-unicorn#adding-unicorn-to-your-application