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
Wtf is this?

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On Thu, Dec 11, 2014 at 6:28 PM, Izaak (John) Alpert
<notifications@github.com> wrote:

>> @@ -273,7 +272,7 @@ def dump_transcript_and_fail(gateway, amount, credit_card, params)
>>        gateway.purchase(amount, credit_card, params)
>>      end
>>  
>> -    File.open("transcript.log", "w") { |f| f.write(gateway.transcript) }
>> +    File.open("transcript.log", "w") { |f| f.write(transcript) }
> same `f.write(@transcript)` ?
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> Reply to this email directly or view it on GitHub:
> https://github.com/Shopify/active_merchant/pull/1488/files#r21725720