Fixed problems with psychometrics (Opaque Keys)

@carsongee Some simple fixes to get psychometrics working again:
- Update key comparisons for Opaque Keys
- Search for 'problem_check' instead of 'save_problem_check' when
  filtering TrackingLogs.
The name of the event type for problem checking was renamed from save_problem_check to problem_check in Oct 2013. 
See: http://edx.readthedocs.org/en/latest/internal_data_formats/tracking_logs.html#problem-check-server