Configure Travis to send emails whenver there is a failed build.
Use an email alias (github-saltfs@servo.org) to ensure all interested
parties can receive emails, instead of just the author and committer.
Per the docs (https://docs.travis-ci.com/user/notifications/), emails
are only sent for branches (e.g. master/nightly/auto).
This is meant for notifications on failed nightly builds, so always
notify on failures but avoid notifying on success.