Added missing sudo from service start/stop

`service` cannot be called as a normal user.

Otherwise you get this very friendly Upstart error message:

    start: Rejected send message, 1 matched rules; type="method_call", sender=":1.282" (uid=1000 pid=18807 comm="start hubot ") interface="com.ubuntu.Upstart0_6.Job" member="Start" error name="(unset)" requested_reply="0" destination="com.ubuntu.Upstart" (uid=0 pid=1 comm="/sbin/init")