The Step parameter is an integer so it's normal that 0.5 doesn't work.
Just bumped in this same problem and worked around it by cloning the whole Rangeslider widget from SilkUI (also, added an extension http://simeydotme.github.io/jQuery-ui-Slider-Pips/#installation).
What I don't understand, is why step is set to be an integer parameter in first place. Underlying implementation is using decimal step value.
Bug/improvement idea considering platform?