Using Yarn with CircleCI and Heroku

circle.yml

machine:
  pre:
    - mkdir ~/.yarn-cache
  node:
    version: stable
dependencies:
  pre:
    - curl -o- -L https://yarnpkg.com/install.sh | bash
  cache_directories:
    - ~/.yarn-cache
  override:
    - yarn install
test:
  override:
    - yarn test
deployment:
  production:
    branch: master
    heroku:
      appname: your-production-app
  staging:
    branch: staging
    heroku:
      appname: your-staging-app

Why can't circle give me an option to make sure I have yarn installed? I don't know. But this worked.