Fixed bug that occurs when range slider is inside a Polymer shadow dom

I was trying to use the range slider inside a Polymer web component, but there was a bug that caused the right-side slider to move the left-side slider when it was clicked.  This is because the d3.event.sourceEvent.target on a drag was being attributed to the overlying shadow DOM, rather than to the slider element. This is likely a bug either in Polymer, D3, or at the browser level, but I fixed it here by removing the need to reference d3.event.sourceEvent.target.