README: Update build badge for github actions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

README.md

 # rust-mavlink
 
-[![Build status](https://travis-ci.org/mavlink/rust-mavlink.svg)](https://travis-ci.org/mavlink/rust-mavlink)
+[![Build status](https://github.com/mavlink/rust-mavlink/actions/workflows/test.yml/badge.svg)](https://github.com/mavlink/rust-mavlink/actions/workflows/test.yml)
 [![Crate info](https://img.shields.io/crates/v/mavlink.svg)](https://crates.io/crates/mavlink)
 [![Documentation](https://docs.rs/mavlink/badge.svg)](https://docs.rs/mavlink)