add github actions CI to README instead of travis (publiclab#8905)

README.md

@@ -2,7 +2,7 @@ PublicLab.org
 ======
 
 [![Code of Conduct](https://img.shields.io/badge/code-of%20conduct-green.svg)](https://publiclab.org/conduct)
-[![Build Status](https://api.travis-ci.com/publiclab/plots2.svg?branch=main)](https://travis-ci.com/publiclab/plots2)
+[![Build Status](https://github.com/publiclab/plots2/workflows/continuous-integration/badge.svg)](https://github.com/publiclab/plots2/actions)
 [![first-timers-only-friendly](http://img.shields.io/badge/first--timers--only-friendly-blue.svg?style=flat-square)](https://code.publiclab.org#r=all)
 [![Join the chat at https://publiclab.org/chat](https://img.shields.io/badge/chat-in%20different%20ways-blue.svg)](https://publiclab.org/chat)
 [![Code Climate](https://codeclimate.com/github/publiclab/plots2/badges/gpa.svg)](https://codeclimate.com/github/publiclab/plots2)