Use getBoundingClientRect for dimensions instead of offsetHeight/offsetWidth

`offsetHeight`/`offsetWidth` does not work for SVG Elements except in Chrome
and this support is being removed from Chrome soon:

>'SVGElement.offsetWidth' is deprecated and will be removed in M50, around
April 2016. See https://www.chromestatus.com/features/5724912467574784 for
more details.

js/lib/d3.dimensions.js

@@ -1,8 +1,10 @@
 d3.selection.prototype.dimensions = function (dimensions) {
     if (!arguments.length) {
         var node = this.node();
-        return [node.offsetWidth,
-                node.offsetHeight];
+        if (!node) return;
+
+        var cr = node.getBoundingClientRect();
+        return [cr.width, cr.height];
     }
     return this.attr({width: dimensions[0], height: dimensions[1]});
 };