slingshot: sync with upstream for upload and moonscript support.

* slingshot: Sync with upstream.
* .travis.yml: Manually merge improvements from slingshot.
* bootstrap: Update from slingshot.
* bootstrap.conf (slingshot_files): Delete removed
ax_compare_version.m4.
* m4/ax_compare_version.m4: New checked in copy, because Zile
configury uses it to check that installed Emacs is new enough.
* .gitignore: Update.

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