upgrade celery to fix memory leak caused by time limits, see http://celery.readthedocs.org/en/3.0/changelog.html
Merge branch 'hotfix/upgrade_celery_to_3.0.25'
Merge tag 'upgrade_celery_to_3.0.25' into dev

Upgrade celery to 3.0.25 to fix memory leak caused by time limit use.

See http://celery.readthedocs.org/en/3.0/changelog.html