Clean apt-cacher-ng cache from expired time-based snapshots

Originally created by @bertagaz on #11532 (Redmine)

During the 2016 June sysadmin shift, we noticed that the apt-cacher-ng cache disk was filling up because of expired time-based snapshots. The deletion of this files should be handled automatically to avoid this situation.

Parent Task: tails/sysadmin#5926