fixes https://github.com/fryn/html5slider/issues/16

the problematic part is `slider.value = value`  (What should that do
anyway? )
I don't see any sense in creating a new slider.value-Object, as it works
fine for me when not doing this.
Actually update the slider when a prop changes
Merge pull request #18 from MaPePeR/master

Fixed #16 "too much recursion"