slingshot: sync with upstream.

* slingshot: Pick up recent improvements, particularly compatibility
with upcoming lyaml 4 release.
* .travis.yml: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: compatibility with lyaml > 3.

lyaml > 3 returns all documents in a YAML stream as a table, not
just the first document.
* lib/specl.in: Detect lyaml 4+ by whether 'require "yaml"'
works, or else wrap the lyaml.load function return value in a
table.
* NEWS: Update.

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