ImportError: cannot import name add_to_builtins

```
File "/Users/erik/env/megalist/lib/python2.7/site-packages/smartextends/__init__.py", line 1, in <module>
    from django.template.loader import add_to_builtin
ImportError: cannot import name add_to_builtins
```

`add_to_builtins` is deprecated and will be removed in Django 1.7
