added yarn to travis
added yarn to travis

Teleopti/styleguide,alinanmorariu/styleguide,blandine/styleguide,tiaoshi/styleguide,GrayOverride/styleguide,blandine/styleguide,blandine/styleguide,alinanmorariu/styleguide,tiaoshi/styleguide,GrayOverride/styleguide,GrayOverride/styleguide,alinanmorariu/styleguide,tiaoshi/styleguide,Teleopti/styleguide