CRM-13885 follow up, set timezone when running cron

Without setting timezone it may send reminders out based on an incorrect timezone assumption - e.g. sending after an event has completed. This has been somewhat intermittantly reported which is confusing as it would seem to affect a large portion of WP sites outside the UK

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* CRM-12523: Wordpress timezone settings not carried to CiviCRM
  http://issues.civicrm.org/jira/browse/CRM-12523