slingshot: sync with upstream, for slack notifications.

* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* bootstrap.conf (slingshot_files):
Remove m4/ax_compare_version.m4.
* .slackid: New file.
* .travis.yml: Regenerate.

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