async is a keyword on 3.7 (elastic#47)

Fixes elastic#44

Author: hroncok

elasticsearch_async/helpers.py

@@ -1,3 +1,4 @@
 import asyncio
 
-ensure_future = getattr(asyncio, 'ensure_future', asyncio.async)
+ensure_future = (getattr(asyncio, 'ensure_future', None) or
+                 getattr(asyncio, 'async', None))