Fix conflicting 0004_ migrations on course_overviews

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`

openedx/core/djangoapps/content/course_overviews/migrations/0004_auto__add_field_courseoverview_enrollment_start__add_field_courseoverv.py → openedx/core/djangoapps/content/course_overviews/migrations/0005_add_enrollment_fields.py

@@ -82,7 +82,7 @@ class Migration(SchemaMigration):
             'has_any_active_web_certificate': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
             'id': ('xmodule_django.models.CourseKeyField', [], {'max_length': '255', 'primary_key': 'True', 'db_index': 'True'}),
             'invitation_only': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
-            'lowest_passing_grade': ('django.db.models.fields.DecimalField', [], {'max_digits': '5', 'decimal_places': '2'}),
+            'lowest_passing_grade': ('django.db.models.fields.DecimalField', [], {'max_digits': '5', 'decimal_places': '2', 'null': 'True'}),
             'max_student_enrollments_allowed': ('django.db.models.fields.IntegerField', [], {'null': 'True'}),
             'mobile_available': ('django.db.models.fields.BooleanField', [], {'default': 'False'}),
             'social_sharing_url': ('django.db.models.fields.TextField', [], {'null': 'True'}),