1.4.1 Heroku Setup

Heroku uses the PostgreSQL database (pronounced “post-gres-cue-ell”, and
often called “Postgres” for short), which means that we need to add the
pg gem in the production environment to allow Rails to talk to Postgres:

group :production do
  gem 'pg', '0.15.1'
  gem 'rails_12factor', '0.0.2'
end

Note also the addition of the rails_12factor gem, which is used by
Heroku to serve static assets such as images and stylesheets.

As mentioned in Section 1.2.4, it’s also a good idea to specify
explictly which version of Ruby our applications expects:

ruby '2.0.0'

(Here I’ve also added the optional RVM gemset line for convenience.