Fix scroll issue in Prototype version

@kenearley @stof 

As first reported (and fixed) by @jeslopalo in #1201. The Prototype and jQuery versions do not use the same logic and the jQuery version appears to have the correct version.

Currently, ``high_top`` is calculated as such.

```coffeescript
//jquery
high_top = @result_highlight.position().top + @search_results.scrollTop()

//prototype
high_top = @result_highlight.positionedOffset().top
```

This change just makes the Prototype version incorporate scrollTop:

```coffeescript
high_top = @result_highlight.positionedOffset().top + @search_results.scrollTop
```