Add gerritlib to requirements
Add gerritlib to requirements

We will need a way to plug-in to the gerrit stream
so that we can push comments; this library seems to
be the defacto one to do this (and I have contributed
code to it so I understand mostly how it works).

Change-Id: I485a7e3a72d8d9042ba6da696808b2e29a8bcb2a

stackforge/nerd-reviewer,openstack/nerd-reviewer