Travis-CI config for Yarn 2

.travis.yml

language: node_js

cache:
  # Yarn 2 does not store dependencies in node_modules anymore, at least not by default.
  # If you are using the node-modules plugin, remove the `npm: false` line.
  npm: false
  # Yarn 2 caches to the local .yarn directory, not the Travis default `$HOME/.yarn` 
  directories:
    - ./.yarn/cache

before_install:
  # Upgrade Yarn 1.x: https://classic.yarnpkg.com/en/docs/install
  - curl -o- -L https://yarnpkg.com/install.sh | bash
  - export PATH="$HOME/.yarn/bin:$PATH"
  # Then the local Yarn 2 should be available and get used
  - yarn -v

# Use custom install step as Travis by default uses the --frozen-lockfile flag, which is deprecated
install: yarn

# rest stays the same, hopefully …

Thanks for this. This helped where official yarn or Travis docs didn't have any information regarding this. I encountered additional problems as Travis was having SSL certificate issues fetching yarn from the URL and yarnrc.yml file making more issues. I changed the before_install as follows and got it working.

before_install:
  - rm .yarnrc.yml
  # Repo for Yarn
  - sudo apt-key adv --fetch-keys http://dl.yarnpkg.com/debian/pubkey.gpg
  - echo "deb http://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
  - sudo apt-get update -qq
  - sudo apt-get install -y -qq yarn
  - yarn set version berry
  - export PATH="$HOME/.yarn/bin:$PATH"
  # Then the local Yarn 2 should be available and get used
  - yarn -v