shell_plus --print-sql not working on Django 1.7

Currently in Django 1.7, the shell_plus --print-sql does not print the sql output.

Looking into the [source code](https://github.com/django/django/blob/stable/1.7.x/django/db/backends/util.py), the `django.db.backends.util` module has been renamed, so use `django.db.backends.utils` instead.