for :production in the Gemfile they use gem 'pg', group: :production

Gemfile

source 'https://rubygems.org'

# Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
gem 'rails', '4.0.1'

# Use sqlite3 as the database for Active Record
gem 'sqlite3', group: [:development, :test]

gem 'rails_12factor', group: :production
gem 'pg', group: :production

# Use SCSS for stylesheets
gem 'sass-rails', '~> 4.0.0'

# Use Uglifier as compressor for JavaScript assets
gem 'uglifier', '>= 1.3.0'

# Use CoffeeScript for .js.coffee assets and views
gem 'coffee-rails', '~> 4.0.0'

# See https://github.com/sstephenson/execjs#readme for more supported runtimes
# gem 'therubyracer', platforms: :ruby

# Use jquery as the JavaScript library
gem 'jquery-rails'

# Turbolinks makes following links in your web application faster. Read more: https://github.com/rails/turbolinks
gem 'turbolinks'

# Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
gem 'jbuilder', '~> 1.2'

group :doc do
  # bundle exec rake doc:rails generates the API under doc/api.
  gem 'sdoc', require: false
end

# Use ActiveModel has_secure_password
# gem 'bcrypt-ruby', '~> 3.1.2'

# Use unicorn as the app server
# gem 'unicorn'

# Use Capistrano for deployment
# gem 'capistrano', group: :development

# Use debugger
# gem 'debugger', group: [:development, :test]

you remove sqlite
you add pg
you change database.yml
then you bundle install

development:
adapter: postgresql
encoding: unicode
database: blog_development
pool: 5
username: nom de ta base de donne
password: password

from OKu7
group :production do
gem 'pg'
gem 'rails_12factor'
gem 'thin'
end
group :development do
gem 'better_errors'
gem 'binding_of_caller', :platforms=>[:mri_19, :mri_20, :rbx]
gem 'quiet_assets'
gem 'rails_layout'
gem 'sqlite3'
end