slingshot: update.

* slingshot: Update.
* configure.ac (SS_CONFIG_TRAVIS): Remove luafilesystem now that
slingshot takes care of ldoc prerequisites.
* .travis.yml: Regenerate.
* .gitignore: Ignore doc directory rather than docs, as latest
ldoc seems to be putting files in the former now.

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