e2e: only run on Go 1.7+ (#244)
e2e: only run on Go 1.7+ (#244)

x/time/rate requires the standard library context. We only run these
tests on Travis using Go 1.8.
danicat/golang-samples,danicat/golang-samples,GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,danicat/golang-samples,adamyanliev/golang-samples,adamyanliev/golang-samples,GoogleCloudPlatform/golang-samples,adamyanliev/golang-samples