Update ci status badge to github actions (#258)

Author: jcrist

README.rst

@@ -8,5 +8,5 @@ Native Kubernetes integration for Dask.
 See documentation for more information:
 `kubernetes.dask.org <https://kubernetes.dask.org/en/latest>`_
 
-.. |Build Status| image:: https://circleci.com/gh/dask/dask-kubernetes.svg?&style=shield
-   :target: https://circleci.com/gh/dask/dask-kubernetes
+.. |Build Status| image:: https://github.com/dask/dask-kubernetes/workflows/CI/badge.svg
+   :target: https://github.com/dask/dask-kubernetes/actions?query=workflow%3ACI