linux-user-chroot replaced by projectatomic/bubblewrap

[Colin Walters <walters verbum org>]

https://git.gnome.org/browse/linux-user-chroot/commit/?id=836ed2249f6ef8a6d0adadf389d3f7951c370363

There aren't yet any official releases of bubblewrap, but we plan to do so soon.  In
the meantime, I'd like to officially deprecate linux-user-chroot in favor of
bubblewrap.

See
https://blogs.gnome.org/alexl/2016/04/29/using-bubblewrap-in-xdg-app/
as well.