Set minimum log level to 'warning' for PostgreSQL

I suggest setting `min_messages` to `warning` for PostgreSQL.

This'll suppress outputs like:

```
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "with_model_genre_tags_947_pkey" for table "with_model_genre_tags_947"
NOTICE:  CREATE TABLE will create implicit sequence "with_model_genre_taggings_947_id_seq" for serial column "with_model_genre_taggings_947.id"
```

These are particularly annoying if you are creating a bunch of tables while running test suite, for example, using `wtih_model` gem.

From the doc (http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html):

```
:min_messages - An optional client min messages that is used in a SET client_min_messages TO <min_messages> call on the connection.
```

See also http://anti-pattern.com/suppressing-notice-messages-with-postgresql-on-rails