message: test(ci): replace node v7.x.x with node v9.x.x
  We only want to test against the latest non-stable version. Also, greenkeeperio/greenkeeper-lockfile#98 is fixed so we can revert the switch to npm.
diff: @@ -7,16 +7,14 @@ cache:
  notifications:
  email: false
  before_install:
  - # TODO: Switch this back to yarn when greenkeeperio/greenkeeper-lockfile#98 is fixed.
  - # https://github.com/greenkeeperio/greenkeeper-lockfile/issues/98
  - - npm install --global greenkeeper-lockfile@1
  + - yarn global add greenkeeper-lockfile@1
  before_script:
  - greenkeeper-lockfile-update
  after_script:
  - greenkeeper-lockfile-upload
  node_js:
  + - '9'
  - '8'
  - - '7'
  - '6'
  - '4'
  after_success:
  