README.md: Update CI badge

We have moved from Travis to GitHub Actions, update build bage
accordingly.

README.md

@@ -7,7 +7,7 @@
 
 Rust (user-space) virtual machine for eBPF
 
-[![Build Status](https://travis-ci.org/qmonnet/rbpf.svg?branch=master)](https://travis-ci.org/qmonnet/rbpf)
+[![Build Status](https://github.com/qmonnet/rbpf/actions/workflows/test.yaml/badge.svg)](https://github.com/qmonnet/rbpf/actions/workflows/test.yaml)
 [![Build status](https://ci.appveyor.com/api/projects/status/ia74coeuhxtrcvsk/branch/master?svg=true)](https://ci.appveyor.com/project/qmonnet/rbpf/branch/master)
 [![Crates.io](https://img.shields.io/crates/v/rbpf.svg)](https://crates.io/crates/rbpf)