Stats cron should be managed by Jenkins

Related to #944 (closed), when the stats cron fails we do not get a notification about it.

Currently stats.civicrm.org has this cron running as root:

0 1 * * * cd /var/www/stats.civicrm.org; ./bin/civicrm-statistics.sh > ./log/civicrm-statitics.log 2>&1

It had been failing for a while, but we were not getting a notification.