Emails should deliver to nobody when perform_deliveries is set to false

The [Rails
Guides](http://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-callbacks)
now recommend using an after_filter and setting perform_deliveries to
false as a method of preventing delivery. It's reasonable to expect that
`expect(email).not_to deliver_to(guest_user)` would work in this
scenario.
Merge pull request #139 from sgrif/sg-perform-deliveries

Emails should deliver to nobody when perform_deliveries is set to false