mwan3-luci: update to 1.3-4

mwan3-luci: update to 1.3-4

corrected name of last_resort option from "main" to "default"
added blackhole option to policy selection on rule configuration
put dummy echo command back in hotplug script send_alert function to avoid errors

Signed-off-by: Aedan Renner <chipdankly@gmail.com>