Add an "any slot" dependency operator for dev-db/postgresql.

This does not change the current behavior, and silences a repoman
warning. I also believe it is correct: dev-db/postgresql is slotted
along major (e.g. 9.3, 9.4) version lines. The libpq interface doesn't
seem to break between those versions, and mailing list discussions by
the Postgres devs indicate that they'd prefer to avoid that kind of
breakage.