What happened:
* First, I merged `0004_default_lowest_passing_grade_to_None.py`, which adds `null=True` to `lowest_passing_grade`.
* Later, Renzo merged `0004_auto__add_field_courseoverview_enrollment_start__add_field_courseoverv.py`, which adds a bunch of enrollment-related fields
* Neither migrations' `models` variables reflects the changes in the other one.
* release currently has just `0004_default...`
* master and rc/2015-07-22 have `0004_auto...` and `0004_default...`, in that order.

What this commit changes:
* Change `0004_auto...` to `0005_add_enrollment_fields.py`
* Update `models` in `0005_...` to have `null=True` for `lowest_passing_grade`