Fix unused property warning in StyleRoot

[kylecesmat]

As of React v15.2.0(facebook/react#6800) - React issues warnings for unknown properties passed to DOM nodes. This should fix the warning when using the StyleRoot component.

I haven't tested this but it seems reasonable.

Fixes #759

[coveralls]

Coverage increased (+0.006%) to 96.266% when pulling 123950b on fix-unknownPropStyleRoot into 7be72ab on master.

[coveralls]

Coverage increased (+0.006%) to 96.266% when pulling a3656bb on fix-unknownPropStyleRoot into 7be72ab on master.

[alexlande]

Thanks @kylecesmat! If appveyor ever catches up, I'll merge this thing.

[conarwelsh]

+1

[sunkaflek]

+1

[alexlande]

This doesn't do the trick, unfortunately. This prop is applied with our _cloneElement wrapper, so we can't handle it in the render method. Furthermore, this happens with all elements processed by Radium, not just StyleRoot. Taking discussion back to the open ticket to figure this out one.