It is often difficult to diagnose new issues server admins get when
upgrading to a new server version because the only information they are
able to tell us regarding the server version they are running is
"latest". This commit attempts to mitigate this by keeping track of the
previous version of Paper they were running, which is then reported by
the `/version` or `/paper version` command. This gives us a better idea
of the commits included in the upgrade, which may help diagnose new
issues easier.
Kyle Wood