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 libmosbus-2.0.x

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