fix crash in peer eval xmodule

As documented in [LMS-806](https://edx-wiki.atlassian.net/browse/LMS-806):

Steps to reproduce:

* as one user, for a peer evaluated problem, enter something that needs peer grading

* as another user, log in and fire up the peer grading panel. You should see something that needs attention, a yellow exclamation badge

* clicking on the "do grading" panel will fail with a 500, stacktrace attached.

* it's possible that the question where this is being done must first have professor calibration done in order for peers to be able to actually get work to grade.

@dmitchell debugged this with me and saw some problems in the ways that the peer_evaluation xmodule code was accessing dates. Instead of using the normal lms accessors, by going straight into the _module_data
it would be bypassing some important code (caching?). Fixing this to be more standard did the trick.

I'd like two reviewers.  @dmitchell please look over.  Should be simple since you and I basically pair-programmed this, but would still like the confirm.  and @VikParuchuri since it was your code originally (I think), and you offered. :smile:.  Thanks.