DOC: Fix build badge from switch to GH Actions.

Author: Scott Sanderson

README.rst

@@ -19,5 +19,5 @@ Installation
    $ pip install python-interface
 
 .. _`documentation` : http://interface.readthedocs.io
-.. |build status| image:: https://travis-ci.org/ssanderson/interface.svg?branch=master
-   :target: https://travis-ci.org/ssanderson/interface
+.. |build status| image:: https://github.com/ssanderson/python-interface/workflows/interface-ci/badge.svg
+   :target: https://github.com/ssanderson/python-interface/actions