Remove depreciation warning during cron job.

Review Request #6300 — Created Sept. 4, 2014 and submitted — Latest diff uploaded

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Review Board
master
7e05f07...

Reviewers

Change crontab entry to use clearsessions in favour of cleanup as
cleanup has been depreciated.

Running ./reviewboard/manage.py cleanup causes warning messages
to be outputted, but running ./reviewboard/manage.py clearsessions
does not.