README: Use gh action badge

See also: https://help.github.com/en/actions/automating-your-workflow-with-github-actions/configuring-a-workflow#adding-a-workflow-status-badge-to-your-repository

Author: tony

README.rst

@@ -214,7 +214,7 @@ tests           .. code-block:: bash
     :alt: Python Package
     :target: http://badge.fury.io/py/libtmux
 
-.. |build-status| image:: https://img.shields.io/travis/tmux-python/libtmux.svg
+.. |build-status| image:: https://github.com/tmux-python/libtmux/workflows/.github/workflows/libtmux-ci.yml/badge.svg
    :alt: Build Status
    :target: https://travis-ci.org/tmux-python/libtmux