trace: skip gRPC interceptor test
trace: skip gRPC interceptor test

It hangs on Go versions less than 1.9.
E.g. See https://travis-ci.org/GoogleCloudPlatform/google-cloud-go/jobs/295701045.

Fixes #801.

Change-Id: I7b4d87d94344c607889de38673c4c2de4e6c8ae4
Reviewed-on: https://code-review.googlesource.com/19152
Reviewed-by: Sarah Adams <2b342d1c32491b4e7587e4a0e291a5f0f95d4167@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go