attempt_stats should exclude anyone in the beta_testers group

@jtriley  Can you take a look at this? I'm trying to get attempt_stats management command into shape for Kerri & Michael. I think excluding the beta tests (like Tej and me) will address some of the discrepancies I'm seeing. 

Also, I assuming I'm submitting my PR to the correct branch. Let me know if I've got that wrong. 

@carsongee FYI