rel-html: update backports stable versions

Add the backports stable version 3.18 and remove 3.15. This should
reflect the current supported versions.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
rel-html: fix url for next signature.

The link to the signature of the next tarball was missing a slash.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>