invoke hook_civicrm_pre for employer relationship on edit

This isn't actually an issue with API but with hooks - might want to create a new component for that.

When the current employer is changed with editing an individual, hook_civicrm_pre is not called for the Relationship change.

Work-around: hook_civicrm_pre is called for the individual, and one can check $objectRef->current_employer against the db's civicrm_contact.employer_id.