Bump PostgreSQL in compose.

[jchristgit]

The commit message contains instructions to upgrade, but here it is for "doppelt hält besser" again, since GitHub nicely pasted it in here:

This might break local PostgreSQL containers with the following error:

postgres_1  | 2019-10-11 20:01:48.772 UTC [1] FATAL:  database files are incompatible with server
postgres_1  | 2019-10-11 20:01:48.772 UTC [1] DETAIL:  The data directory was initialized by PostgreSQL version 11, which is not compatible with this version 12.0.

You will need to do one of the following:

- Delete the old data volume and start from scratch (simple, recommended).
  As we don't have an explicit volume configured, only the one the PostgreSQL
  image uses, this is as simple as running `docker-compose stop` followed by
  `docker-compose rm`. Afterwards, you can restart it.

- The dump & restore route. Do this if you don't want to delete your
  container data. To do this, first switch the PostgreSQL version in
  `docker-compose.yml` back to `11`, then run the following commands:

    docker-compose stop                                         # Stop all containers
    docker-compose start postgres                               # Start the database
    docker-compose exec postgres pg_dumpall -U pysite > db.sql  # Pull a SQL dump
    docker-compose stop                                         # Stop the containers again
    docker-compose rm                                           # Remove the containers

  Now, update the PostgreSQL version in `docker-compose.yml` to `12` again,
  and run the following commands:

    docker-compose up -d postgres                               # Start the database
    docker-compose exec -T postgres psql -U pysite < db.sql     # Pull in the dump
    rm db.sql                                                   # Cleanup garbage, if you want

  If you're on Windows, then I can't help you. Sorry.

See also: https://github.com/python-discord/organisation/issues/149