Run travis tests against node 5
Run travis tests against node 5

Fixes #129

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