Requiring _just_ the `Aws::SNS::MessageVerifier` does not work as it
references other classes in the AWS SDK that are not in this one class.
Bringing in the entire SNS SDK verifies the authenticity correctly.
```
rails/actionmailbox$ bundle exec rake tests
(snip)
...
rails/activerecord/lib/active_record/persistence.rb:48: warning: in `create!':
the last argument was passed as a single Hash
rails/actionmailbox/app/models/action_mailbox/inbound_email/message_id.rb:21:
warning: although a splat keyword arguments here
...
(snip)
```
