Update build status badge in README.md (#74)

Currently it points to no longer used travis-ci. This PR changes it to
GitHub test workflow.

Author: zero323

README.md

@@ -2,7 +2,7 @@
 
 # pytest plugin for testing mypy types, stubs, and plugins
 
-[![Build Status](https://travis-ci.org/typeddjango/pytest-mypy-plugins.svg?branch=master)](https://travis-ci.org/typeddjango/pytest-mypy-plugins)
+[![Tests Status](https://github.com/typeddjango/pytest-mypy-plugins/actions/workflows/test.yml/badge.svg)](https://github.com/typeddjango/pytest-mypy-plugins/actions/workflows/test.yml)
 [![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)