mwan3: update to version 1.5-6

Fixed issue where mwan3 would not immediately set interface down on link-loss event
Added feature to define last-resort action on policies with no members

Signed-off-by: Jeroen Louwes <jeroen.louwes@gmail.com>
Merge pull request #354 from Adze1502/master

mwan3: update to version 1.5-6