Register PostgreSQL citext type to prevent "unknown OID" warnings

on citext columns.

Citext is a PostgreSQL data type for case-insensitive character strings.

To use it in a Rails application, add the `citext` extension in a migration:

````
    class AddCitextExtension < ActiveRecord::Migration
      def up
        execute ‘CREATE EXTENSION IF NOT EXISTS citext;’
        add_column :users, :search_field, 'citext'
      end

      def down
        remove_column :users, :search_field, 'citext'
        execute ‘DROP EXTENSION IF EXISTS citext CASCADE;’
      end
    end
````