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Summary:
Per instructions at
http://docs.travis-ci.com/user/notifications/#Slack-notifications

Test Plan: fingers crossed.

Reviewers: mroth, emily

Reviewed By: mroth, emily

Subscribers: benjaminpollack

Differential Revision: https://phabricator.khanacademy.org/D21940

ariabuckles/perseus,ariabuckles/perseus,learningequality/perseus,learningequality/perseus,ariabuckles/perseus,ariabuckles/perseus