Update db/structure.sql to new format

As of Rails 4.0, `db:structure:dump` now uses `mysqldump` rather than a
custom format:

    https://github.com/rails/rails/commit/ccc6910cb034efc6b749e1ae82c748085a671fa9

It now includes a lot more detail - this commit doesn't include any
actual schema changes.
This could be exciting when @rgarner triggers a change to `structure.sql`...