Rendering layout blocks fails when using django-model-utils 2.0+ with Django 1.5.5

[roadsideseb]

The test rendering block subclasses currently fails when running against django-model-utils 2.0+ and Django 1.5.5. It reaches maximum recursion depth in when trying to render layout blocks. This needs further investigation to be fixed. For the time being, version 1.5 of django-model-utils seems to fix the problem (at least in the tests).