Fix MySqlAdapter superclass bug via class_eval loading of superclasses

To get it working with my current setup (a lot of gem dependencies) and Rails 3.2.13.

Removing the `superclass mismatch for class MysqlAdapter` by `class_eval` as opposed to monkey patching the superclass.

It was still erroring for me with the fix in #185. Would be great if @etagwerker (or others where this is an issue) could check if this fix also works for them.