Timeouts when using the feature spec helpers under poltergeist

[maeve]

@jonnysamps reported timeouts when executing examples that use the auth feature spec helpers in conjunction with the poltergeist driver (i.e. example groups tagged with :auth and :js simultaneously). This is not a case that is currently covered by g5_authenticatable's own test suite, which uses the racktest driver for all feature specs. However, it is a scenario we should support if at all possible.

As a temporary workaround, the :auth_request helpers can be used in javascript feature specs instead. This works, but it drops us directly into warden instead of using the omniauth test mode. It does not really help client applications assert that they've configured their before filters correctly.