update travis to call yarn
update travis to call yarn

I purposefully left `npm` in charge of calling running the scripts, as
we're primarily interested in using yarn for package installation.

StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,carls-app/carls,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,carls-app/carls,carls-app/carls,StoDevX/AAO-React-Native,carls-app/carls,StoDevX/AAO-React-Native,StoDevX/AAO-React-Native,carls-app/carls,carls-app/carls,carls-app/carls