use operation.promise() instead of after-operation

[DrPaulBrewer]

Probably should now use operation.promise() instead of after-operation

Example: Deleting a VM in Google Compute Engine

// other code in function to identify zone, name of vm to delete
return gce.zone(zone).vm(name).delete().then(
      ([op, apiResponse]) => (op.promise()),
      (e)=>(console.log('error on delete vm '+name+':'+e))
    );