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.
Merge pull request #144 from stripe/better-rakefile

Run tests using TestTask