Doesn't seem to work with Rails 4

Rails 4 seem to use the `PG::Connection#connect` method rather than `PG#connect`. If I add the following monkey-patch, the queries do seem to execute asynchronously:

```rb
module PG
  class Connection
    def self.connect(*args)
      Async::Postgres::Proxy.new(*args)
    end
  end
end
```

I can send a PR in a few weeks, if that's the way we want to go.