Handle Travis CI Events at /event/travis_ci

Without this patch we have no facility to receive and handle events
generated by the Travis CI webhook.  This patch lays the groundwork,
performing authentication of the request made by TravisCI.

The relevant configuration for participating repositories is:

    notifications:
    email: false
    webhooks:
      urls:
        - https://puppet-dev-community.herokuapp.com/event/travis_ci/
      on_success: always
      on_failure: always
      on_start: true

Webhook notification documentation is available at:
http://about.travis-ci.org/docs/user/notifications/#Webhook-notification