Update python-protobuf to python3-protobuf

m prereqs (or rather install-rereqs.sh) was getting
stuck on an error, unable to locate package `python-protobuf`, and not
install remaining dependencies.

Updating python-protobuf -> python3-protobuf which apt is able to
locate.

This also allows the remainder of the script to run and install
remaining prereqs.

Change-Id: If4c376d0c9139d02b6a95291770ca97b4b4dc7fd