Support for ActiveRecord schema format `:sql` in migrations

Enable support for the `:sql` schema format in Rails. See [ActiveRecord::Base.schema_format](http://api.rubyonrails.org/classes/ActiveRecord/Base.html#method-c-schema_format) for details on `schema_format`.

For example, this is useful for Postgres databases using `hstore`.

_NOTE_
This method currently does _not_ work for _multi-schema_ databases, since the schema names are usually embedded in the `structure.sql` file.