Use Virtus for Pandarus models

Pandarus now uses Virtus to define its models. This simplifies
a lot of the model base class, and offers several nice features
as well.

- Update APIs to canvas-lms 042dfb3233 (with changes)
- bump to 0.6.0

Fixes SIS-644, SIS-623

Change-Id: I1b548e12953a274f2dd7f587fff06b10945ded63