README: GH actions badge

Author: MuellerSeb

README.md

@@ -2,7 +2,7 @@
 
 [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.1229051.svg)](https://doi.org/10.5281/zenodo.1229051)
 [![PyPI version](https://badge.fury.io/py/welltestpy.svg)](https://badge.fury.io/py/welltestpy)
-[![Build Status](https://travis-ci.com/GeoStat-Framework/welltestpy.svg?branch=develop)](https://travis-ci.com/GeoStat-Framework/welltestpy)
+[![Build Status](https://github.com/GeoStat-Framework/welltestpy/workflows/Continuous%20Integration/badge.svg?branch=develop)](https://github.com/GeoStat-Framework/welltestpy/actions)
 [![Coverage Status](https://coveralls.io/repos/github/GeoStat-Framework/welltestpy/badge.svg?branch=develop)](https://coveralls.io/github/GeoStat-Framework/welltestpy?branch=develop)
 [![Documentation Status](https://readthedocs.org/projects/welltestpy/badge/?version=latest)](https://geostat-framework.readthedocs.io/projects/welltestpy/en/latest/?badge=latest)
 [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)