Bypass race condition

[swelham]

There looks to be a race condition in the paypal tests that occasionally causes the following error.

No HTTP request arrived at Bypass
     stacktrace:
       (bypass) lib/bypass.ex:19: anonymous fn/1 in Bypass.open/1
       (ex_unit) lib/ex_unit/on_exit_handler.ex:98: ExUnit.OnExitHandler.exec_callback/1
       (ex_unit) lib/ex_unit/on_exit_handler.ex:82: ExUnit.OnExitHandler.on_exit_runner_loop/0

This needs to be investigated and fixed.