Merge pull request #884 from redboltz/fix_gha_bardge

Replaced travis-ci badge with github actions badge.

Author: redboltz

README.md

@@ -1,7 +1,7 @@
 `msgpack` for C++
 ===================
 
-Version 3.3.0 [![Build Status](https://travis-ci.org/msgpack/msgpack-c.svg?branch=cpp_master)](https://travis-ci.org/msgpack/msgpack-c) [![Build status](https://ci.appveyor.com/api/projects/status/8kstcgt79qj123mw/branch/cpp_master?svg=true)](https://ci.appveyor.com/project/redboltz/msgpack-c/branch/cpp_master)
+Version 3.3.0 [![Build Status](https://github.com/msgpack/msgpack-c/workflows/CI/badge.svg?branch=cpp_master)](https://github.com/msgpack/msgpack-c/actions) [![Build status](https://ci.appveyor.com/api/projects/status/8kstcgt79qj123mw/branch/cpp_master?svg=true)](https://ci.appveyor.com/project/redboltz/msgpack-c/branch/cpp_master)
 [![codecov](https://codecov.io/gh/msgpack/msgpack-c/branch/cpp_master/graph/badge.svg)](https://codecov.io/gh/msgpack/msgpack-c/branch/cpp_master)
 
 It's like JSON but smaller and faster.