Travis: consider running CI on beta, nightly, and tor's lowest supported rust
At the moment, Tor's Travis CI runs on stable rust, and our Appveyor (Windows) CI doesn't run rust at all (#26954 (moved)).
But in our privcount_shamir work (#25669 (moved)), we've discovered some important bugs by running beta and nightly.
Since Rust releases every 6 weeks, we should run CI on rust stable and beta, so that we catch any show-stoppers before they are stable.
We might also want to consider an allow_failures nightly build.
If we choose to support a lower version of rust than stable (when a major distro freezes on a lower version), we should also CI that version.