stable branch uses an obsolete APT snapshot => FTBFS

Originally created by @intrigeri on #12038 (Redmine)

11:28:13 E: Failed getting release file http://time-based.snapshots.deb.tails.boum.org/debian/2016092002/dists/jessie/Release

Seems that the Valid-Until of this snapshot wasn’t updated when 2.8 was canceled and replaced with 2.9. I’ll let the current RM decide what’s the best way to deal with this.

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