Fix drag icons on taxonomy page.

#6266 

Changed the CSS property ``background-position-y`` to ``background-position`` as it is not working in Firefox.

Drag icons on chrome:
![selection_274](https://cloud.githubusercontent.com/assets/4497748/3395580/9995b934-fd00-11e3-98b5-3442d1f7d0f7.png)

Drag icons on Firefox:
![selection_275](https://cloud.githubusercontent.com/assets/4497748/3395579/95302f64-fd00-11e3-8e1e-05159ea0273a.png)