Travis Build fails due to the migration of CI environment

Created by: tharakawj

Travis CI environment has been migrated from Ubuntu Precise to Ubuntu Trusty which has yarn v0.17.8 pre installed.

This cause lerna bootstrap.js file to use yarn and fails as yarn v0.17.8 doesn't support --non-interactive flag.

https://travis-ci.org/facebookincubator/create-react-app/jobs/270969063#L499

I think there are two options to solve this. We can either explicitly set dist: precise in .travis.yml until they update yarn to a latest version or change bootstrap.js to check USE_YARN in shouldUseYarn method.

Not sure what's the better approach.