clusters_diff: remove the hard dependency on numpy (#41)
clusters_diff: remove the hard dependency on numpy (#41)

This makes travis cry because numpy takes ages to compile and
isn't strictly needed to have something functional.
unbrice/biggraphite,dpanth3r/biggraphite,iksaif/biggraphite,iksaif/biggraphite,Thib17/biggraphite,iksaif/biggraphite,criteo/biggraphite,criteo/biggraphite,natbraun/biggraphite,criteo/biggraphite,Thib17/biggraphite,Thib17/biggraphite,Thib17/biggraphite,iksaif/biggraphite,criteo/biggraphite