Update build status link for django-stubs-ext (#2310)

Author: flaeppe

ext/README.md

@@ -1,6 +1,6 @@
 # Extensions and monkey-patching for django-stubs
 
-[![Build Status](https://travis-ci.com/typeddjango/django-stubs.svg?branch=master)](https://travis-ci.com/typeddjango/django-stubs)
+[![Build status](https://github.com/typeddjango/django-stubs/workflows/test/badge.svg?branch=master&event=push)](https://github.com/typeddjango/django-stubs/actions?query=workflow%3Atest)
 [![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
 [![Gitter](https://badges.gitter.im/mypy-django/Lobby.svg)](https://gitter.im/mypy-django/Lobby)