README: Replace Travis CI badge with GHA
This was supposed to have been done with the move to GitHub Actions:
Better late than never
# Exercism Rust Track
-[![Build Status](https://travis-ci.org/exercism/rust.svg?branch=master)](https://travis-ci.org/exercism/rust)
+[![CI](https://github.com/exercism/rust/workflows/CI/badge.svg?branch=master)](https://github.com/exercism/rust/actions?query=workflow%3ACI+branch%3Amaster)
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Exercism exercises in Rust
