Fix outdated workflow badge

README.md

 # The Rust standard library's portable SIMD API
-[![Build Status](https://travis-ci.com/rust-lang/portable-simd.svg?branch=master)](https://travis-ci.com/rust-lang/portable-simd)
+![Build Status](https://github.com/rust-lang/portable-simd/actions/workflows/ci.yml/badge.svg?branch=master)
 
 Code repository for the [Portable SIMD Project Group](https://github.com/rust-lang/project-portable-simd).
 Please refer to [CONTRIBUTING.md](./CONTRIBUTING.md) for our contributing guidelines.