Version 1.5 of OpenTripPlanner was released just before version 2.0. It is intended to serve as a final release integrating any bugfixes backported from the 2.x development as the 1.x track moves into legacy/maintenance status. OpenTripPlanner 2 has changed significantly from OTP1 so there are reasons you may still want to work with this 1.x release or a fork of it. See http://docs.opentripplanner.org/en/latest/Version-Comparison/ for more information.