Fix django < 1.5 smart_text import

Sorry, looks like I overlooked this in 899b3092ffc696981d66da986883bc407b6e3d89.

Thanks to @shultais for [the report](https://github.com/ecometrica/django-dbarray/commit/899b3092ffc696981d66da986883bc407b6e3d89#commitcomment-5150241).