I’m running Ubuntu 14.04.3, and the unity-editor-5.1.0f3+2015082501_amd64 package has a dependency on lib32stdc++6 that can’t be easily resolved without significant downgrades.
The root of the problem seems to be that it creates a dependency chain that would force a downgrade of the gcc-4.9-base package from 4.9.2 to 4.9.1.
I haven’t run all the way down that rabbithole yet because I’m concerned about causing instability on my work machine, but at my earliest opportunity I’ll see if I can figure out what’s what on another machine and report back, unless someone else already has a solution.
I do have Universe and Multiverse enabled, so maybe that is affecting things?