eval_cached event: change interface to traceID\tjobsetID\tevaluationID

I was not going to break the interface until I noticed
the current implementation uses the string literal \t.

src/script/hydra-notify

@@ -98,10 +98,12 @@ $listener->subscribe("build_queued");
 $listener->subscribe("build_started");
 $listener->subscribe("cached_build_finished");
 $listener->subscribe("cached_build_queued");
+$listener->subscribe("eval_cached");
 $listener->subscribe("eval_started");
 $listener->subscribe("hydra_notify_dump_metrics");
 $listener->subscribe("step_finished");
 
+
 # Process builds that finished while hydra-notify wasn't running.
 for my $build ($db->resultset('Builds')->search(
                    { notificationpendingsince => { '!=', undef } }))