heroku_gemfile.rb

# making your Gemfile safe for Heroku
ruby '1.9.3'   # just in case - tell Heroku which Ruby version we need
group :development, :test do
  # make sure sqlite3 gem ONLY occurs inside development & test groups
  gem 'sqlite3' # use SQLite only in development and testing
end
group :production do
  # make sure the following gems are in your production group:
  gem 'pg'              # use PostgreSQL in production (Heroku)
  gem 'rails_12factor'  # Heroku-specific production settings
end