Since cbee084ac1, this playbook supports Postgres 10.x,
but keeps existing Postgres-9.x installs on 9.x.
This playbook can now also be ran with `--tags=upgrade-postgres`
to make it upgrade from Postgres 9.x to 10.x (or other versions
in the future).
The non-working script is supposed to be fixed
by https://github.com/matrix-org/synapse/pull/2375
To have it work, we'd need an updated Docker image
of `silviof/matrix-riot-docker:latest`, which is not yet available
at the time of this commit.
Still, the previous patched synapse_port_db didn't work well either,
so it's not like we're regressing much by getting rid of it.