Use github-actions build badge instead of travis (#147)

README.md

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 # Jupyter Server
 
 [![Google Group](https://img.shields.io/badge/-Google%20Group-lightgrey.svg)](https://groups.google.com/forum/#!forum/jupyter)
-[![Build Status](https://travis-ci.org/jupyter/jupyter_server.svg?branch=master)](https://travis-ci.org/jupyter/jupyter_server)
+[![Build Status](https://github.com/jupyter/jupyter_server/workflows/CI/badge.svg)](https://github.com/jupyter/jupyter_server/actions)
 [![Documentation Status](https://readthedocs.org/projects/jupyter-server/badge/?version=latest)](http://jupyter-server.readthedocs.io/en/latest/?badge=latest)
 
 The Jupyter Server provides the backend for Jupyter web applications such as