Make Rake task db:schema:load compatible with SQL schema format

If you in your environment.rb have set "config.active_record.schema_format = :sql" then the Rake task db:schema:load (and the upcoming db:setup task in Rails 3 that depends on it) won't work for you. As it stands, db:schema:load will always attempt to load a Ruby format schema file regardless of your schema format setting. This patch changes db:schema:load to load from db/development_structure.sql if schema format is :sql.