Postgres Adapter broken for Postgres >9

I guess lighthouse is too full to find important tickets.

But as I see it,
this is quite an important issue to fix.
(so worth shouting on here about)

Notably with version 9.0.1 (the current stable version) of Postgres
Rails incorrectly returns

supports_disable_referential_integrity? false

consequently test fixtures do not work when using foreign keys.

https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/5635-postgresql_adapter-doesnt-disable-referential-integrity-for-pg-90#ticket-5635-5
https://github.com/rails/rails/pull/122