slingshot: sync with upstream, for rockspecs in $buildreq.

* slingshot: Sync with upstream.
* bootstrap: Sync with slingshot.
* configure.ac (SPECL_MIN): Remove.
* bootstrap.conf (buildreq): Add ldoc and specl rockspecs.
* .travis.yml: Regenerate, but manually splice in the LDoc-1.4.0
luarocks install while waiting for a luarocks release.

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