Fix binary treemap pivot.

We were doing binary search, but we weren’t checking whether the previous pivot
is closer to the target value than the following pivot. Related #71.

src/treemap/binary.js

@@ -28,6 +28,8 @@ export default function(parent, x0, y0, x1, y1) {
       else hi = mid;
     }
 
+    if ((valueTarget - sums[k - 1]) < (sums[k] - valueTarget) && i + 1 < k) --k;
+
     var valueLeft = sums[k] - valueOffset,
         valueRight = value - valueLeft;