The do-release-upgrade abort the upgrade process if you have one of the following packages installed:
It looks like these packages are not available on 16.04 (though they are available for PostgreSQL 9.5) bit they are matching a removal black list pattern.
Simple remove these packages manually to proceed with upgrade. You may reinstall them after upgrading Ubuntu to 16.04 and PostgreSQL to 9.5.
A bug report has been filled, please vote if it affects you.