The pop-up on https://www.postgresql.org/download/linux/ubuntu/ still lists Ubuntu Wily (15.10), which is no longer supported as of 2016-07-28. Today, Ubuntu Yakkety (16.10) was released, so it seems like it might be a good time to retire 15.10 in favor of a set of 16.10 packages.
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Jason Petersen
Software Engineer | Citus Data
303.736.9255
jason(at)citusdata(dot)com