Use OneToOneField instead of ForeignKey(unique=True)

It's cleaner approach to define the same relation.
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/migrations/0020_auto_20150220_1356.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('trans', '0019_auto_20150220_1354'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='indexupdate',
+            name='unit',
+            field=models.OneToOneField(to='trans.Unit'),
+            preserve_default=True,
+        ),
+    ]

weblate/trans/models/search.py

@@ -22,7 +22,7 @@ from django.db import models
 
 
 class IndexUpdate(models.Model):
-    unit = models.ForeignKey('Unit', unique=True)
+    unit = models.OneToOneField('Unit')
     source = models.BooleanField(default=True)
 
     class Meta(object):