change the value of angleIndicator to make it work in ie11

"none" is a invalid value to strokeStyle in ie,so black will be automatically set to strokeStyle.In other browsers "none" is legal and equal to "hsla(0, 0%, 0%, 0)".So I think "hsla(0, 0%, 0%, 0)" may be more appropriate here.
Merge pull request #17 from icemoegirl/patch-2

change the value of angleIndicator to make it work in ie11