math/gringo: Update 4.5.2 -> 4.5.4

Port changes:

• Maintainer reset based on 6.5 months timeout
• Use DISTVERSION
• Add LICENSE_FILE
• Add USES=python:2.7
• Remove '+' from '+=' where they aren't needed
• Update WWW to anew address

PR: 225131
Submitted by: Nathan <ndowens.fbsd@yandex.com>
Approved by: vsevolod@FreeBSD.org (maintainer's timeout 6.5 months)