Update build badge to use github workflow CI

Author: piotrszul

README.md

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 # Variant Spark
 
-[![Travis-Build](https://travis-ci.org/aehrc/VariantSpark.svg?branch=master)](https://travis-ci.org/aehrc/VariantSpark#)
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 _variant-spark_ is a scalable toolkit for genome-wide association studies optimized for GWAS like datasets.