databases/postgresql-plproxy: Update from 2.5 to 2.7

Changelog: https://plproxy.github.io/changelog.html

This adds support for PostgreSQL 9.5

Also:

• take maintainership
• change MASTERSITE to GitHub
• remove obsoleted check for PostgreSQL 9.1 (no longer in ports)
• update homepage of project

PR: 201796, 217515