Fixed return object from subscriber.subscribe() (#1685)

pubsub/cloud-client/subscriber.py

@@ -255,13 +255,13 @@ def callback(message):
         print('Received message: {}'.format(message))
         message.ack()
 
-    subscription = subscriber.subscribe(subscription_path, callback=callback)
+    future = subscriber.subscribe(subscription_path, callback=callback)
 
     # Blocks the thread while messages are coming in through the stream. Any
     # exceptions that crop up on the thread will be set on the future.
     try:
         # When timeout is unspecified, the result method waits indefinitely.
-        subscription.future.result(timeout=30)
+        future.result(timeout=30)
     except Exception as e:
         print(
             'Listening for messages on {} threw an Exception: {}.'.format(