Update requirements for Django 1.5+

Using these updated requirements, I was able to run [pyohio/pyohio](https://github.com/pyohio/pyohio) locally using Django 1.6.

Running through the release notes, I noticed no major breaking changes. However, `taggit` did introduce migrations::

```
django-admin.py migrate taggit 0001 --fake
django-admin.py migrate
```