readme: update status badge for GitHub Actions instead of Travis

README.md

@@ -1,4 +1,4 @@
-[![Build Status](https://travis-ci.org/panda3d/panda3d.svg?branch=master)](https://travis-ci.org/panda3d/panda3d)
+[![Build Status](https://github.com/panda3d/panda3d/workflows/Continuous%20Integration/badge.svg?branch=master)](https://github.com/panda3d/panda3d/actions?query=branch%3Amaster+workflow%3A%22Continuous+Integration%22)
 [![OpenCollective](https://opencollective.com/panda3d/backers/badge.svg)](https://opencollective.com/panda3d)
 [![OpenCollective](https://opencollective.com/panda3d/sponsors/badge.svg)](https://opencollective.com/panda3d)