Add support for pre-release grpc v0.15
Add support for pre-release grpc v0.15

googleapis/cloud-trace-nodejs,keymetrics/vxx,keymetrics/trassingue,matthewloring/cloud-trace-nodejs,keymetrics/vxx,googleapis/cloud-trace-nodejs,googleapis/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,matthewloring/cloud-trace-nodejs,googleapis/cloud-trace-nodejs,keymetrics/trassingue