fix default sort so it sorts by number instead of order opened

[thasmin]

This prevents buffers from being listed out of order. It happens to me pretty often in two situations, but I'm sure there's more. One is when buffers are closed and then reopened and the other is when buffers are opened in the background via vimgrep.

[ursuscamp]

It's a genuine shame this plugin seems to have fallen out of maintenance, because this change by thasmin is fantastic and needs to be merged.