Switch Travis badge to GitHub Actions
.. image:: https://img.shields.io/codecov/c/github/beetbox/beets.svg
:target: https://codecov.io/github/beetbox/beets
-.. image:: https://travis-ci.org/beetbox/beets.svg?branch=master
- :target: https://travis-ci.org/beetbox/beets
+.. image:: https://github.com/beetbox/beets/workflows/ci/badge.svg?branch=master
+ :target: https://github.com/beetbox/beets/actions
.. image:: https://repology.org/badge/tiny-repos/beets.svg
:target: https://repology.org/project/beets/versions
