aria2-1.21.0 has already been released. Please update portage. See https://github.com/tatsuhiro-t/aria2/releases for the latest changes.
commit c07a10fee955bc389b8ada0db44fbd82d25397d4 Author: Michał Górny <mgorny@gentoo.org> Date: Sat Apr 9 12:42:14 2016 net-misc/aria2: Bump to 1.21.0, #579022