Possible Issue for Django 1.6 in django_pgjson 0.3.0

Hi,

in fields.py there is this import 

"from django.utils.module_loading import import_string"

but in Django 1.6 import_string is not present.

My current solution is :
if django.get_version() >= "1.7":
    from django.utils.module_loading import import_string
else:
    from django.utils.module_loading import import_by_path as import_string

Please let me know your opinion. Thx a lot.
