Merge pull request #138 from pusher/support-python37

Mention we support Python 3.7 in the README

Author: WillSewell

README.md

@@ -8,7 +8,7 @@ In order to use this library, you need to have a free account on <http://pusher.
 
 ## Features
 
-* Python 2.7, 3.4, 3.5 and 3.6 support
+* Python 2.7, 3.4, 3.5, 3.6 and 3.7 support
 * Adapters for various http libraries like requests, urlfetch, aiohttp (requires Python >= 3.5.3) and tornado.
 * WebHook validation
 * Signature generation for socket subscriptions