проблема с развертыванием rails приложения на heroku

в Gemfile

group :development do
  # Access an IRB console on exception pages or by using <%= console %> anywhere in the code.
  gem 'web-console'
  gem 'listen', '~> 3.0.5'
  gem 'sqlite3'
  # Spring speeds up development by keeping your application running in the background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'
end

group :production do
    gem 'pg'
    gem 'rails_12factor'
end    <pre>

database.yml

<pre>    development:
  adapter: sqlite3
  database: db/development.sqlite3
  pool: 5
  timeout: 5000

# Warning: The database defined as "test" will be erased and
# re-generated from your development database when you run "rake".
# Do not set this db to the same as development or production.
test:
  adapter: sqlite3
  database: db/test.sqlite3
  pool: 5
  timeout: 5000

production:
  adapter: postgresql
  database: liceum
  encoding: utf8
  reconnect: true

когда выполняю git push heroku master

выдает

remote:        An error occurred while installing sqlite3 (1.3.13), and Bundler cannot
remote:        continue.
remote:        Make sure that `gem install sqlite3 -v '1.3.13'` succeeds before bundling.
remote:  !
remote:  !     Failed to install gems via Bundler.
remote:  !     Detected sqlite3 gem which is not supported on Heroku:
remote:  !     https://devcenter.heroku.com/articles/sqlite3

почему так происходит, ведь для продакшина прописан postgres ?????