django 1.5.5 backwards incompatible changes

Django 1.5.5 introduced some backwards incompatible changes.

Rather than workaround this I've opted just to support 1.5.5 and later. Is this okay?

Previously we were supporting django>=1.2 but I'm not sure Mathics would even run on django==1.2.
Either way I think we should at least drop django<=1.3

Contains a fix for https://groups.google.com/forum/#!topic/mathics-users/gYPx-zgZ1tE.