Upgrade django-markitup from 1.0.0 -> 2.1

For Django 1.6 compatibility.
Update django-reversion from 1.7 -> 1.8

For Django 1.6 compatibility.
Update django-sitetree from 0.9.4 -> 1.0.0

For Django 1.6 compatibility.
Update django-taggit from 0.9.3 -> 0.11.2

For Django 1.6 compatibility.
Update easy-thumbnails from 1.2 -> 1.4

For Django 1.6 compatibility.
Loosen Django requirement

So that each project may control which version of Django to use. Not all
of the latest requirements are compatible with Django 1.4 anymore.
Merge pull request #7 from pyohio/update-requirements

Update requirements for Django 1.5+