fix ie bug on categoryAxis.ts

IE 9 was failing on category axes. This branch fixes the bug.

Specifically, it was failing http://jsfiddle.net/DDwz8/23/ because IE 9 doesn't support `instanceof` on "arrays that are actually objects", like d3 selections or jQuery selections.

Note that cramped category axes still look ugly, but that's a separate issue.
Good dodge of the `instanceof` problem on IE9. We should file a ticket to check other parts of our codebase for that.