From http://devcenter.wercker.com/articles/werckeryml/

wercker.yml

box: wercker/ruby
services:
    - wercker/postgresql
build:
    steps:
        # Execute the bundle install step, a step provided by wercker
        - bundle-install
        - rails-database-yml:
            service: postgresql
        # Execute a custom script step.
        - script:
            name: echo ruby information
            code: |
              echo "ruby version $(ruby --version) running!"
              echo "from location $(which ruby)"
              echo -p "gem list: $(gem list)"
        - script:
            name: Set up db
            code: RAILS_ENV=test bundle exec rake db:schema:load
        - script:
            name: Run RSpec
            code: bundle exec rspecdeploy:
deploy:
    steps:
        - heroku-deploy
        - script:
            name: Update database
            code: heroku run rake db:migrate --app $APP_NAME