if 'libapt' is not the root cause and if it already is installed on your system , you can probably update safely using `aptitude update`.
Interestingly both are given "urgency=low" ratings; at least 3 other updates have been medium urgency this year.
Edit: sorry should have said on Ubuntu; I've got libapt-inst1.5:amd64 (1.0.1ubuntu2.3) from trusty/main.