Remove deprecated call to "load firstof from future"

[azaghal]

The syntax load firstof from future in template zinnia_bootstrap/templates/comments/zinnia/entry/form.html has been deprecated since Django version 1.8, and has been completely removed from Django 1.10.

When using the theme with Django 1.10, the following error is reported when comment form is rendered:

'future' is not a registered tag library. Must be one of:

It should be sufficient to remove {% load firstof from future %} to fix compatibility with Django 1.10.

[azaghal]

As per #22, I can create a pull request for this one if you're interested :)

[Fantomas42]

Closed by #24