slingshot: sync with upstream, for Lua 5.3.0 improvements.

* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* NEWS: Move from here...
* NEWS.md: ...to here. Convert to Markdown format.
* .gitignore: Add NEWS.
* local.mk (old_NEWS_hash): Regenerate.
(luaexec_LTLIBRARIES): Change from assigment to appending.
* .travis.yml: Regenerate.

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