Switch back to NPM until TravisCI build includes Yarn pre-installed.
Switch back to NPM until TravisCI build includes Yarn pre-installed.

- https://github.com/travis-ci/travis-ci/issues/6720#issuecomment-254988120

WillowDeploy/willow-deploy,WillowDeploy/willow-deploy,WillowDeploy/willow-deploy,WillowDeploy/willow-deploy