Avoid using a server version which is newer than pg_dump

This handles generally the problem with Debian based distributions where pg_dump may be older than one or more of the server versions in `/usr/lib/postgresql`, by only considering server versions that are equal to or older than pg_dump.  It also allows an environment variable to override where M::B::DB::PostgreSQL looks for the server binaries, in case you want to use a specific version.

In my testing I could see that the `/usr/lib/postgresql` on travis had multiple versions.  I couldn't figure out why the failures weren't deterministic, I wasn't able to reproduce that.  Perhaps the VMs had subtle differences in the installed packages?