Add migrations due to field default changes from #10802.

Django 1.8 is more sensitive to model changes, and wants to create
migrations in situations that Django 1.4 would not. Changing field
defaults is an example of these situations.

common/djangoapps/student/migrations/0002_auto_20151208_1034.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('student', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='courseenrollment',
+            name='mode',
+            field=models.CharField(default=b'audit', max_length=100),
+        ),
+        migrations.AlterField(
+            model_name='historicalcourseenrollment',
+            name='mode',
+            field=models.CharField(default=b'audit', max_length=100),
+        ),
+    ]

lms/djangoapps/shoppingcart/migrations/0002_auto_20151208_1034.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('shoppingcart', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='courseregcodeitem',
+            name='mode',
+            field=models.SlugField(default=b'audit'),
+        ),
+        migrations.AlterField(
+            model_name='paidcourseregistration',
+            name='mode',
+            field=models.SlugField(default=b'audit'),
+        ),
+    ]