Currently certbot v0.8.0 is the latest.
I merely renamed app-crypt/acme-0.6.0 and app-crypt/certbot-0.6.0 ebuilds to their *-0.8.0 pendants, generated the digests and after that, emerging and then using these versions just worked for me.
0.8.1 is released now.
Author: Manuel Rüger <email@example.com>
Date: Sat Jul 9 15:47:35 2016 +0200
app-crypt/certbot: Version bump to 0.8.1