www/py-django-assets: change the requirement in requires.txt for webassets to
be at least version 0.10 instead of exactly version 0.10

Bump PORTREVISION

This allows the (work in progress) Pootle port to show a usage line instead of
a dependency error.

Differential Revision: https://reviews.freebsd.org/D2928
Approved by: maintainer (wg)