The olders 4.2.8 version is no longer available from the Schneider
Electric website. It has been replaced with "PowerNet MIB v4.5.1"
which can be downloaded from this page:
https://www.se.com/us/en/download/document/APC_POWERNETMIB_451_EN/



Fix the download link accordingly.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>