relay: show version on startup

Since relay is a long-running process, I find myself in situations where I dont know which version of croc I have running.

This simple startup log will make it obvious.

src/cli/cli.go

@@ -380,6 +380,7 @@ func receive(c *cli.Context) (err error) {
 }
 
 func relay(c *cli.Context) (err error) {
+	log.Infof("starting croc relay version %v", Version)
 	debugString := "info"
 	if c.GlobalBool("debug") {
 		debugString = "debug"