patch rails 4 schema.rb generation to include sharded tables

When rails generates db/schema.rb, it previously did not include any sharded tables. This adds an additional `ActiveRecord::Schema.define` block populated with the sharded tables. The sharded database will receive its own `schema_migrations` table when running `rake db:schema:load`.

This behavior currently only occurs on Rails 4+ / Ruby 2+

FWIW, it's a copy straight from ActiveRecord: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/schema_dumper.rb#L59