libmodbus: bump version and change download location

The license of libmodbus is LGPL v2.1 or later and the
licence of programs in tests directory is GPL v3. So
specify only LGPL v2.1.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>