Fitted the importer to new json output by hektor

Also changed the length restriction of the StudentInfo.matrikel_no to 30

core/migrations/0009_auto_20180320_1922.py

+# Generated by Django 2.0.3 on 2018-03-20 19:22
+
+import core.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0008_auto_20180219_1712'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='studentinfo',
+            name='matrikel_no',
+            field=models.CharField(default=core.models.random_matrikel_no, max_length=30, unique=True),
+        ),
+    ]