issue: ticket.dept.sla Variable sla_id
This addresses an issue where when using the variable ticket.dept.sla the
system throws a fatal error. This is due to the SLA's relation to Department
defined in include/class.dept.php. It's defined as array('sla_id' => 'SLA.sla_id') which is incorrect as SLA.sla_id does not exist. This
updates the relation from array('sla_id' => 'SLA.sla_id') to
array('sla_id' => 'SLA.id') so that it uses the appropriate column.