It turns out the memory leak was not directly caused by the Django upgrade or Celery.
After a lot of digging around I found that, surprisingly, the celery worker memory leak happens because I upgraded django-debug-toolbar from 0.9.4
to 0.11.0
(which is needed for Django 1.6 compatibility).
Still no idea what exactly caused this issue, or why it only happens in the celery worker processes and not in the app server ones (Gunicorn).
Removing django-debug-toolbar
from the installed apps and middleware solves the issue. At least temporarily.