slingshot: sync with upstream.

* slingshot: Sync with upstream, for release.mk fixes.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: require not-yet-released Specl 12 for Larch specs.

* .travis.yml (script): Temporarily install specl master.
* bootstrap.conf (buildreq): Require (unreleased!) specl 12.

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