Update authentication.md

Based on the [new documentation](https://docs.djangoproject.com/en/dev/topics/auth/customizing/#django.contrib.auth.get_user_model) for Django 1.7 we should now use:

```python
@receiver(post_save, sender=settings.AUTH_USER_MODEL)
```
instead of:

```python 
@receiver(post_save, sender=get_user_model())
```
because `get_user_model()` only works once Django has imported all models. Otherwise you'll get: 

```python
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.
```
When trying to start your sever after adding token authentication.

From the Docs, ( linked above as well ) :
>New in Django 1.7:
>When connecting to signals sent by the User model, you should specify the custom model using the AUTH_USER_MODEL setting.
Merge pull request #1997 from agconti/patch-1

docs(api-guide/authentication): updated to reflect new standards in Django 1.7