docs: replace build status badge in the index file of the docs
TreeTime: time-tree and ancestral sequence inference
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-.. image:: https://travis-ci.org/neherlab/treetime.svg?branch=master
- :target: https://travis-ci.org/neherlab/treetime
+.. image:: https://github.com/neherlab/treetime/actions/workflows/ci.yml/badge.svg?branch=master
+ :target: https://github.com/neherlab/treetime/actions/workflows/ci.yml
.. image:: https://anaconda.org/bioconda/treetime/badges/installer/conda.svg
:target: https://anaconda.org/bioconda/treetime
