BK-992 Publishing engine does not work with latest version of Celery

This fix the problem about new celery versions and also the problem for AI.
I've tested this on Ubuntu 14.04 and Mac Yosemite with django-celery==3.0.23 and celery 3.0.25, django-celery==3.0.23 and celery 3.023, django-celery==3.1.16 and celery==3.1.16 (lastest stable versions). 

I did not tried with PDF rendering yet.