Update Document model's uuid field to use Django's native UUIDField class.

mayan/apps/documents/migrations/0031_convert_uuid.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+import uuid
+
+from django.db import migrations, models
+
+
+def convert_uuid_to_hex(apps, schema_editor):
+    Document = apps.get_model('documents', 'Document')
+
+    for document in Document.objects.all():
+        document.uuid = uuid.UUID(document.uuid).hex
+        document.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0030_auto_20160309_1837'),
+    ]
+
+    operations = [
+        migrations.RunPython(convert_uuid_to_hex),
+    ]