It needs to be rebuild when >=sys-power/upower-0.9.23 subslot is bumped, I would add ":=" then for upower dep :) Thanks
*mate-power-manager-1.8.1 (09 Dec 2014) 09 Dec 2014; Matthias Maier <tamiko@gentoo.org> +mate-power-manager-1.8.1.ebuild: version bump; fix libgnome-keyring dependency wrt bug #522798; fix subslot dependency wrt bug #523826; fix unique configuration wrt bug #514176