README: Replace Travis CI badge with GHA (#1000)
README: Replace Travis CI badge with GHA (#1000)

This was supposed to have been done with the move to GitHub Actions:
https://github.com/exercism/rust/pull/975
https://github.com/exercism/rust/issues/974
https://github.com/exercism/rust/issues/982

Better late than never
petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust