Use relative path in sorting_tags.py

Author: stephane

webstack_django_sorting/templatetags/sorting_tags.py

@@ -5,7 +5,8 @@
 from django.conf import settings
 from django.http import Http404
 from django.utils.translation import ugettext_lazy as _
-from webstack_django_sorting.util import get_sort_field
+
+from ..util import get_sort_field
 
 
 register = template.Library()