Disable slider, if iOSNativeScrolling is used and supported

Hello,

This PR hides slider if iOSNativeScrolling is used and supported. Otherwise both nanoscroller and native scrollers are shown.

Couldn't compile full build due to grunt-css [bug](https://github.com/jzaefferer/grunt-css/issues/25) (or it's just me?)

Let me know what you think about it.