Forks filebrowser to fix tests

We can't downgrade filebrowser, because <3.5.7 have tests that fail on
django>=1.4.14. Instead, fork 3.5.7 with the compatibility issue
reverted, until they fix it.

An issue has been filed for the compatibility issue. See
<https://github.com/sehmaschine/django-filebrowser/issues/241>.

See also the revert made in the fork, at
<https://github.com/learning-unlimited/django-filebrowser/commit/4557e6a6>.

Corrects add9c22d.