Update to redis-rb 3.1.0; replace #unknown_items transaction with a script

This PR gets the tracker on redis-rb 3.1.0 (i.e. latest stable release), and updates all Redis code to match.  Updating to the latest release made implementing the changes for #20 much easier.  These changes make the tracker spec pass using redis-rb 3.1.0.

This PR also replaces the `multi` block in `Transactions#unknown_items` with a script, because that was one transaction I thought would be clearer as a script.  AFAICT, the script has exactly the same effect and offers the same atomicity guarantees.