Fix versioning for realsies

[GhostofGoes]

• Add Unix timestamp to version to ensure packages get ordered by time, since apt will install the latest after sorting alphabetically.
• Fix zmq install in pybennu. Yeah, it was using pip instead of pip3, so this might have been broken for a while and no one noticed. I added set -e to the install script to ensure it exits if any of the commands fail in the future
• Move postinstall into a separate script, instead of inline in the Makefile
• Add a script to build C++ bennu
• Some minor cleanup