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