security/py-certbot: update to 0.13.0

• Update PORTVERSION and distinfo checksum (0.13.0)
• Update pkg-descr to cope with the renaming to "certbot"

Changes: https://github.com/certbot/certbot/blob/master/CHANGELOG.md#0130---2017-04-06

Approved by: koobs (maintainer)
Differential Revision: https://reviews.freebsd.org/D10317