README: add github workflows CI badge
# Altair <a href="https://altair-viz.github.io/"><img align="right" src="https://altair-viz.github.io/_static/altair-logo-light.png" height="50"></img></a>
[![build status](http://img.shields.io/travis/altair-viz/altair/master.svg?style=flat)](https://travis-ci.org/altair-viz/altair)
+[![github actions](https://github.com/altair-viz/altair/workflows/build/badge.svg)](https://github.com/altair-viz/altair/actions?query=workflow%3Abuild)
[![JOSS Paper](http://joss.theoj.org/papers/10.21105/joss.01057/status.svg)](http://joss.theoj.org/papers/10.21105/joss.01057)
[![PyPI - Downloads](https://img.shields.io/pypi/dm/altair)](https://pypi.org/project/altair)
[![Binder](https://mybinder.org/badge_logo.svg)](https://mybinder.org/v2/gh/altair-viz/altair_notebooks/master?urlpath=lab/tree/notebooks/Index.ipynb)
