Avoid the assignment_tag deprecation warning (django-webpack#88)

django-countries does it the same way, see SmileyChris/django-countries@0477bdb56

Author: matthiask

webpack_loader/templatetags/webpack_loader.py

@@ -1,4 +1,4 @@
-from django import template
+from django import template, VERSION
 from django.conf import settings
 from django.utils.safestring import mark_safe
 
@@ -50,7 +50,8 @@ def webpack_static(asset_name, config='DEFAULT'):
  )
 
 
-@register.assignment_tag
+assignment_tag = register.simple_tag if VERSION >= (1, 9) else register.assignment_tag
+@assignment_tag
 def get_files(bundle_name, extension=None, config='DEFAULT'):
  """
  Returns all chunks in the given bundle.