Transactions should not be joinable

I believe this fixes #234. I documented my repro steps at rails/rails#12944. This is based off the [Rails 4 sandbox code](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/railties/console_sandbox.rb), so I'm not sure what would be needed to fix this on 3.2.

If you want to add tests or investigate further etc, feel free to close this and fix it via another commit.