travis: slack: Fix 'on_success' to be 'change'
travis: slack: Fix 'on_success' to be 'change'

That is, remove extra colon introduced in 75ecf73 ("travis: Adjust
Slack notification preferences") and suggested in
travis-ci/travis-ci#2894 as a workaround.

Let's stick to the official way:

    https://docs.travis-ci.com/user/notifications/#Slack-notifications

Also add the explicit setting for the 'on_failure' preference and
regenerate the secure token.

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