Refresh fix

[Isuru-Nanayakkara]

Fixes the issue #36.

Chose @AndreaStevanato's fork because it's up to date with Swift 4 and has some added functionality.

[prilaga]

Hi. Unfortunately, the changes are missed in the release version 1.8.0
I can't refresh the slider programmatically. I need it for example when I want to reset this view.

But I found another approach.
In order to refresh programmatically I used next:

slider.selectedMinValue = 0
slider.selectedMaxValue = 500
slider.layoutSubviews()