Pass right timestamp instead of crap from string(int64)

Problem is that --since can consume anything not near looking as
timestamp.

Signed-off-by: Alexandr Morozov <lk4d4@docker.com>

integration-cli/docker_cli_events_test.go

@@ -425,7 +425,7 @@ func TestEventsStreaming(t *testing.T) {
 	eventDestroy := make(chan struct{})
 
 	go func() {
-		eventsCmd := exec.Command(dockerBinary, "events", "--since", string(start))
+		eventsCmd := exec.Command(dockerBinary, "events", "--since", strconv.FormatInt(start, 10))
 		stdout, err := eventsCmd.StdoutPipe()
 		if err != nil {
 			t.Fatal(err)