sysutils/vagrant: patch gemspec for rubygem-listen 2.8.0 update

• Allow vagrant to run with devel/rubygem-listen 2.8.0 as of r373273.

PR: 195588
Differential Revision: https://reviews.freebsd.org/D1273
Submitted by: Kevin Bowling <kevin.bowling@kev009.com>
Approved by: joe@thrallingpenguin.com (maintainer)
Approved by: koobs (mentor)