fix exception when using django extensions' find_template command

[miraculixx]

exception was as follows

# with django-extensions installed, this is Django 1.7
$ manage.py find_template app/template.html
(...)
raise NotImplementedError('subclasses of BaseLoader must provide a
load_template_source() method')
NotImplementedError: subclasses of BaseLoader must provide a load_template_source() method

[craiglabenz]

Hey @miraculixx -- apologies for going code here! I don't remember ever seeing an alert for this PR. Are you still dealing with this? I've recently bumped django-tablets to support only django>=1.8.

[miraculixx]

@craiglabenz indeed, this fix is in production use. However, I'm still using a tablets version at Django<1.8. I'll be happy to rebase and update, but will be a while...