remove extra Experiment.find
fix bug with returning the winner
Re-order alternative choosing precedence

New precedence ordering:
* The force param
* If the server is not enabled, the control is returned
* If there's a winner, it's returned
* If the visitor is excluded, return the control
* Otherwise create an internal client_id and return a "chosen" alternative

This ensures the following:
* Bots do not cause internal client_ids to be created
* Bots *do* get the winner if one exists
* The force param *always* works
* Redis work is minimized

Note: I added code to delete all sixpack related keys before starting the tests. I don't *think* there's anything wrong with that, but I figured I'd call it out.
Experiment.winner is now a cached property
duplicate conversions aren't exceptional