language: node_js
node_js:
  - '9.11.2'
before_install:
  - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.10.1
  - export PATH="$HOME/.yarn/bin:$PATH"
  - npm install -g webpack
install:
  - npm install
  - npm run build
script:
  - npm run build-test
  - npm run coverage
after_success: 'npm run coveralls'
cache:
  yarn: true
  directories:
    - node_modules

language: node_js
node_js:
  - '9.11.2'
before_install:
  - npm install -g webpack
install:
  - yarn install
  - yarn run build
script:
  - yarn run build-test
  - yarn run coverage
after_success: 'npm run coveralls'
cache:
  yarn: true
  directories:
    - node_modules

Change commands to use yarn instead of npm
Change commands to use yarn instead of npm