Fixes SwiftMailer exception reported by TravisCI

This PR addresses #143. TravisCI is disallowing the test-suite from sending mail over SMTP as part of the "Speaker submits a new talk" use-case.  This contains a set of refactors to afford ability to mock the mailer call as part of the TalkControllerTest.

- Adds config to application container.
- Extracts SwiftMailer reference into application container at key, 'mailer'.
- Updates to TalkControllerTest to mock call to mailer send method.