[Koha] Upgrading from 17.05.00.00 to "current"

Does anyone have any tips on how to successfully go from Koha 17.05 to anything more current on Debian 8 (Jessie) package install? I've tried the process I've used in the past and the other OS packages upgrade but Koha keeps getting "held back" no matter what I've tried (various 'koha.list' sources, apt-get full-upgrade, apt-get dist-upgrade, etc.). What is the latest version that will run on Debian 8 and what should I put in my koha.list source file? Thanks! Steve