Reduce ActiveRecord connection pool from 5 to 2

We're lowering the ActiveRecord connection pool maximum from the default
of 5. A basic Rails app with 2 Heroku web dynos that each have 3 Unicorn
workers, plus two Delayed Job background workers could have up to 40
database connections if ActiveRecord or our code is causing hung
connections.

While the Rails 4 reaper will close dead connections every 10 seconds,
limiting the total to 1 per process (8 in this case), we still maintain
some Rails 3 apps. Also, while it is likely that over time our
application may have a few connections that hang, if something in our
code is causing hung connections, the reaper will not be a permanent fix
to the problem.

Depending on the pricing plan, production Heroku Postgres databases have
a maximum of 60 to 500 connections. That sounds like a lot, but it's
likely that before the max is hit, we'll run into some other resource
contention like CPU, IO throughput, or memory. Database throughput will
hit a local max and then degrade as number of connections increase.

2 is a good default as opposed to 1 because New Relic's auto-EXPLAIN
feature also adds 1 connection per Unicorn (or background job).

https://devcenter.heroku.com/articles/concurrency-and-database-connections