FIX requirements.txt

Msg de erro gerado no Travis:

```
pip install -r requirements.txt --use-mirrors

--use-mirrors has been deprecated and will be removed in the future.
Explicit uses of --index-url and/or --extra-index-url is suggested.

--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
```
Merge pull request #921 from jfunez/fix_requirements

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