force registration of overextends tag library

this fixes sporadic import error by importing get_template  ?!?!?!
most likely a race condition of some sort

see:
https://github.com/django-compressor/django-compressor/issues/131
https://github.com/django-compressor/django-compressor/issues/141
Merge branch 'master' of https://github.com/henzk/django-productline