Switch transcript capturing to use wiredump_device; Update scrub tests to not use send

It turns out capturing transcripts is easier than what I was doing before; you can use [wiredump_device](https://github.com/Shopify/active_utils/blob/master/lib/active_utils/common/connection.rb#L104) from active_utils.

@edward @ntalbott @karlhungus for review
I've found this to be confusing before, is transcript a local var, or is it @transcript? i _think_ it's a locally scoped var, should it be `@transcript = ''`?