mwan3: update to version 1.5-4

Fix issue #219: Policy status output returns arithmetic syntax error

Signed-off-by: Jeroen Louwes <jeroen.louwes@gmail.com>
libmodbus: revert to 2.0.4 to unbreak collectd support

collectd is currently the only user of libmodbus.
Thus include the version of libmodbus which allows collectd
modbus plugin to build.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
collectd: modbus support works with libmodbus-2.0.x

Signed-off-by: Daniel Golle <daniel@makrotopia.org>