Resolve "Received 500 on export Student Scores"

Closes #140 (closed)

core/migrations/0012_auto_20190306_1611.py

+# Generated by Django 2.1.5 on 2019-03-06 16:11
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0011_auto_20181001_1259'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='studentinfo',
+            name='exam',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='students', to='core.ExamType'),
+        ),
+    ]