Replace build status badge

Signed-off-by: falkTX <falktx@falktx.com>

README.md

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 # DPF - DISTRHO Plugin Framework
-[![Build Status](https://travis-ci.org/DISTRHO/DPF.png)](https://travis-ci.org/DISTRHO/DPF)
+[![Build Status](https://github.com/DISTRHO/DPF/actions/workflows/makefile.yml/badge.svg)](https://github.com/DISTRHO/DPF/actions/workflows/makefile.yml)
 
 DPF is designed to make development of new plugins an easy and enjoyable task.<br/>
 It allows developers to create plugins with custom UIs using a simple C++ API.<br/>