dns/knot1: update 1.6.7 -> 1.6.8

Changes: https://www.knot-dns.cz/2016-08-08-version-168-lts.html

Approved by: ports-secteam (with hat)
Security: CVE-2016-6171
Security: https://vuxml.FreeBSD.org/freebsd/7d08e608-5e95-11e6-b334-002590263bf5.html
MFH: 2016Q3