Remove use of deprecated function.

guardian/models/models.py

@@ -3,7 +3,7 @@
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ValidationError
 from django.db import models
-from django.utils.translation import ugettext_lazy as _
+from django.utils.translation import gettext_lazy as _
 from guardian.compat import user_model_label
 from guardian.ctypes import get_content_type
 from guardian.managers import GroupObjectPermissionManager, UserObjectPermissionManager