Enable Travis CI Slack Notifications

This enables Travis CI to notify Slack the token is semi-secret
so there's no real need to encrypt it. As stated in the Slack docs:

"Your integration token is semi-secret. The worst someone could do
if they had it was post Travis payloads to your channel. They could
not read any data."
Merge pull request #90 from opscode/slack-travis-ci-notifications-64844284

Enable Travis CI Slack Notifications