Update net-mgmt/prometheus to 1.6.1, add version info
Removed STRIP=, as since go 1.6 stripping go binaries doesn't break them.
Details of changes in this release are available here:
https://github.com/prometheus/prometheus/releases/tag/v1.6.1
PR: 218737
Submitted by: ports@ecadlabs.com (maintainer)
Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D10423