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

install-prereqs.sh

diff --git a/install-prereqs.sh b/install-prereqs.sh
index 15f6936..6335b46 100755
--- a/install-prereqs.sh
+++ b/install-prereqs.sh
@@ -59,7 +59,7 @@
     perl
     protobuf-compiler
     python-dev
-    python-protobuf
+    python3-protobuf
     python3
     python3-dev
     python3-pip