Don't try to EXPLAIN select_db calls

I'm using the mysql2 adapter and my app switches back and forth between a few databases in order to do sharding. I've been getting these errors: `ActiveRecord::StatementInvalid: Mysql2::Error: Table '<dbname>.select_db' doesn't exist: EXPLAIN select_db <dbname>`.

This pull requests fixes the problem caused by an overly fuzzy matching of the SQL commands that can be explained.