ENH/BUG: update default threshold for _compute_rank

@agramfort @Eric89GXL @mluessi FYI
BTW this is an old version of Maxfilter, you should use 2.2 which has tSSS info, p.21:

http://imaging.mrc-cbu.cam.ac.uk/meg/Maxfilter_V2.2?action=AttachFile&do=get&target=Maxfilter_Manual_v2pt2.pdf