Have travis test against Go 1.5 (not tip)
Have travis test against Go 1.5 (not tip)

Change-Id: I553f6e5c67b1d6b19634ea7c3891017fcdef30f7
Reviewed-on: https://code-review.googlesource.com/3763
Reviewed-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

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