Run tests using TestTask
Run tests using TestTask

- Runs all tests at once instead of one at a time, this way we don't pay
the cost of constantly reloading the environment. Takes test time from 20
seconds to 4.

- Removes the separate testing using activesupport/all, which wasn't actually
doing anything since the tests were running in a separate process.

Thanks to @JoshCheek for surfacing these issues.

finbarr/kickstarter_stripe,OutlawAndy/stripe-ruby,alkesh26/stripe-ruby,HashNuke/stripe-ruby,hmcfletch/stripe-ruby,iloveitaly/stripe-ruby,theicfire/stripe-ruby,ajporterfield/stripe-ruby,molawson/stripe-ruby,thestorefront/stripe-ruby,PivotDesk/stripe-ruby,nbrustein/stripe-ruby,avvo/stripe-ruby,Easybring/stripe-ruby,gizm0duck/stripe-ruby,stripe/stripe-ruby