Please fix divide by 0

[kthacker7]

In TTRangeSlider.m, line 315, you have:

if (self.enableStep && self.step>=0.0f){
_selectedMinimum = roundf(self.selectedMinimum/self.step)_self.step;
_selectedMaximum = roundf(self.selectedMaximum/self.step)_self.step;
}

If self.step == 0.0, then there is a divide by zero setting selectMinimum and selectMaximum to NaN.
Hope this helped.