FIX requirements.txt: django-htmlmin.git is not the right format; it must have #egg

Msg de erro gerado no Travis:

```
pip install -r requirements.txt --use-mirrors
...
--editable=git+git://github.com/scieloorg/django-htmlmin.git is not the
right format; it must have #egg=Package

Storing debug log for failure in /home/travis/.pip/pip.log
```