Update the CI status badges

Use the badges from GitHub Actions.

README.md

@@ -1,5 +1,7 @@
-[![Build Status](https://travis-ci.org/anholt/libepoxy.svg?branch=master)](https://travis-ci.org/anholt/libepoxy)
-[![Build status](https://ci.appveyor.com/api/projects/status/xv6y5jurt5v5ngjx/branch/master?svg=true)](https://ci.appveyor.com/project/ebassi/libepoxy/branch/master)
+![Ubuntu](https://github.com/anholt/libepoxy/workflows/Ubuntu/badge.svg)
+![macOS](https://github.com/anholt/libepoxy/workflows/macOS/badge.svg)
+![MSVC Build](https://github.com/anholt/libepoxy/workflows/MSVC%20Build/badge.svg)
+![MSYS2 Build](https://github.com/anholt/libepoxy/workflows/MSYS2%20Build/badge.svg)
 
 Epoxy is a library for handling OpenGL function pointer management for
 you.