Update CI badge in README from travis to GH actions

Author: WillSewell

README.md

@@ -1,6 +1,6 @@
 # Pusher Channels HTTP Python Library
 
-[![Build Status](https://travis-ci.org/pusher/pusher-http-python.svg?branch=master)](https://travis-ci.org/pusher/pusher-http-python)
+[![Build Status](https://github.com/pusher/pusher-http-python/workflows/Tests/badge.svg)](https://github.com/pusher/pusher-http-python/actions?query=workflow%3ATests+branch%3Amaster)
 
 This package lets you trigger events to your client and query the state of your channels. When used with a server, you can validate webhooks and authenticate `private-` or `presence-` channels.