Update APIs to use new Google cloud paths.

common/gcloud/pubsub/scopes.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package pubsub
 
 import (
-        "google.golang.org/cloud/pubsub"
+        "cloud.google.com/go/pubsub"
 )
 
 var (
         // PublisherScopes is the set of OAuth2 scopes needed for a publisher to
         // publish messages.
         PublisherScopes = []string{
                 pubsub.ScopePubSub,
         }
 
         // SubscriberScopes is the set of OAuth2 scopes needed for a subscriber to
         // pull and acknowledge messages.
         SubscriberScopes = []string{
                 pubsub.ScopePubSub,
         }
 )