Looks like shotwell upstream is at 0.20.0 (we missed 0.19.0 too!). Simple rename of the 0.18.1 ebuild seems to be fine, there are no dep changes in the diff from 0.18.1->0.20.0.
*shotwell-0.20.0 (20 Sep 2014) + + 20 Sep 2014; Justin Lecher <jlec@gentoo.org> +shotwell-0.20.0.ebuild: + Version BUmp, #523288; drop old +