message: Improve CI.
  Add build for osx.
  Build against latest stable node.js.
  Use yarn cache.
diff: language: node_js
  -node_js: "6"
  +os:
  + - linux
  + - osx
  +node_js:
  + - "node"
  + - "6"
  +cache: yarn
  script: npm run build
  # See this if we need to test the extension via vscode:
  # https://code.visualstudio.com/Docs/extensions/testing-extensions#_running-tests-automatically-on-travis-ci-build-machines
  