slingshot: sync with upstream for upload and moonscript support.

* slingshot: Sync with upstream.
* bootstrap.conf (slingshot_files): Delete removed
ax_compare_version.m4.
* README.md (Installation): Show moonscript rocks repo.
* .travis.yml: Regenerate.

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

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