Places tasks update

The aim of this is to improve the reliability of the places importers; and in particular do not switch to production incomplete data (i.e. rather serve slightly stale data than incomplete). Also provide a better handling of exceptions in the chain of subtasks (see http://celery.readthedocs.org/en/latest/userguide/canvas.html#chord-errors)
* [x] Celery updated
* [x] Importer OxPoints organisation descendants should be removed from import_all and run separately (doesn't affect the places index so it's rather confusing to have it running in the same process)
* [x] Document ordering of importers (now OxPoints is running first due to conflicts in POIs and primary IDs)