Since 3.1, the Mercator projection does not clamp y-values. For rendering
latitudes near the pole, the y-values can approach infinity. Rather than
clamping each point in the projection, d3.geo.mercator now sets the clipExtent
automatically (if not overridden) using the projection's scale and translate.

We'll still use the major-versioned name file for d3js.org, but this is simpler.