www/py-django-mptt: Update to 0.7.4, Fix *_DEPENDS

This change is made to fix the fact that the current version
of this port (0.6.0) is not compatible with Django 1.8, the
port that it RUN_DEPENDS on. It is broken at run-time in both
Ports HEAD and Quarterly.

A soon to be committed update to www/py-django-dpaste (D4048) requires
this update to also fix it being broken in the quarterly branch.

The only other consumer of this port www/py-django-cms, already
incorrectly depending on versions of Django and django-mptt that are
later than those specified in its setup.py:

django-mptt>=0.5.1,<0.5.3',
'Django>=1.4,<1.6',

• Update PORTVERSION and distinfo checksum (0.7.4)
• Update COMMENT to match upstream (setup.py:description=)
• Add LICENSE_FILE
• Django is a RUN not BUILD depends, remove accordingly
• Sort USE_PYTHON values (portlint)
• Enable achitecture independent (NO_ARCH)

Changes:

https://github.com/django-mptt/django-mptt/compare/0.6.0...0.7.0

[1] https://svnweb.freebsd.org/changeset/ports/400452

Reviewed by: antoine
Approved by: portmgr (antoine)
MFH: 2015Q4 (pre-approved: antoine)
Differential Revision: https://reviews.freebsd.org/D4024