Replace Travis CI badge by GitHub CI

README.md

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 A groovy modbus library
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-[![Build Status](https://travis-ci.org/stephane/libmodbus.svg?branch=master)](https://travis-ci.org/stephane/libmodbus)
+![Build Status](https://github.com/stephane/libmodbus/actions/workflows/build.yml/badge.svg)
 
 Overview
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