Renamed the subscriber auto subscribe method, due to changes in the S…

…ubserver PR

See:
- lifechurch/subserver#17
- lifechurch/subserver@8d2ebbe
- lifechurch/subserver@a59caef

event-notification/lib/icalia/event/subscriber.rb

@@ -43,7 +43,7 @@ def subscription_name
         "-#{topic_name}"
       end
 
-      def ensure_subscription_exists
+      def auto_subscribe
         client = Subserver::Pubsub.client
         topic = client.topic(topic_name) || client.create_topic(topic_name)