travis: support slack.com notifications.

* m4/slingshot.m4 (SS_CONFIG_TRAVIS): If there is a .slackid
file, use it to append a slack notification to .travis.yml.
* .slackid: New file.
* .travis.yml: Regenerate.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>