message: Use modern Yarn version on Travis CI
  Travis CI uses a quite old version of Yarn by default. This adds Yarn's
  recommended incantation for using the latest stable version.
diff: @@ -13,6 +13,9 @@ node_js:
  - node # Latest stable version of nodejs.
  addons:
  chrome: stable
  +before_install:
  + - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.13.0
  + - export PATH=$HOME/.yarn/bin:$PATH
  install:
  - ./scripts/travis/install-deps.sh
  matrix:
  