longrunning: use 1.6 context
longrunning: use 1.6 context

Use golang.org/x/net/context for Go 1.6 compatibility.

Change-Id: If1e22369498914a871bf247b42dc6e65c06c1161
Reviewed-on: https://code-review.googlesource.com/8570
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go