Minimal wercker.yml for Heroku deploy

wercker.yml

box: ubuntu
build:
    steps:
        - script:
            name: build
            code: echo build

deploy: 
    steps:
        - script:
            name: install git
            code: sudo apt-get install curl git -y

        - heroku-deploy:
            key: $HEROKU_API_KEY
            app-name: $HEROKU_APP_NAME
            user: $HEROKU_DEPLOY_USER