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. You
should substitute ’1.9.3’ if that’s the version of Ruby you’re using,
though for this tutorial the difference shouldn’t ever matter.) Applying
these changes to the Gemfile from Listing 1.5 yields Listing 1.9.