Use new installer feature from packager.io

Postinstall for openproject is now in its own addon at https://github.com/pkgr/addon-openproject.

Conflicts:

	.pkgr.yml
Add addon-legacy-installer
Removal of builpack entry
Add ruby 2.3.1 to setup
Merge pull request #2076 from opf/feature/packager-new-installer-stable

Use new packager installer