Fix for Python3

I was getting errors on
```python
{% load markdown_deux_tags %}
```
telling me it was getting an import error on "force_unicode".

Some digging in the docs led me to:
https://docs.djangoproject.com/en/1.6/ref/utils/

Which said:
> force_unicode
> Historical name of force_text(). Only available under Python 2.

So based on the use of the word "historical", I changed it to use force_text.