Updating `yarn` in CircleCI

circle.yml

general:
  artifacts:
    - "log"
    - "npm-debug.log"
    - "yarn-error.log"

machine:
  node:
    version: 6.10.1

dependencies:
  pre:
    - echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > ~/.npmrc
    - mkdir -p ~/.cache/yarn
    # This seems to have problems with CircleCI cache and only takes ~15 seconds so we could always download.
    - if [[ ! -e /usr/share/yarn/bin/yarn ]]; then curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add - && echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list && sudo apt-get update && sudo apt-get install yarn=0.24.6-1; fi
  cache_directories:
    - /usr/share/yarn/bin
    # This is to be specific to the installed `yarn` modules repo. We can leave out `v1` to be more futureproof.
    - ~/.cache/yarn/v1
  override:
    # We need to update `$PATH` anywhere else this would be used since Circle has problems setting it globally.
    - /usr/share/yarn/bin/yarn install