Update README.md to reflect Spanner @ GA (#2794)
Update README.md to reflect Spanner @ GA (#2794)


jgeewax/gcloud-node,googleapis/google-cloud-node,googleapis/google-cloud-node,GoogleCloudPlatform/gcloud-node,jgeewax/gcloud-node,GoogleCloudPlatform/gcloud-node,GoogleCloudPlatform/gcloud-node,googleapis/google-cloud-node,googleapis/google-cloud-node