Compatibility with d3-scale@3

[Fil]

see #29: we are blocked at d3-scale@2.1.2 — I think it's because d3/d3-scale#117 has changed the way scales work with a degenerate domain. Not sure if we need to change the specs/tests* or find a way around the change.

Ref. https://github.com/romsson/d3-gridding/blob/master/test/coordinate-test.js#L12