Added missing key props

Summary: This was an error appearing in the console that was bugging me. About how arrays of React nodes should have a key prop.

Reviewed By: passy

Differential Revision: D16646487

fbshipit-source-id: b61841c001eb4a93f0a071557b921ca7365e2ac3

src/chrome/RatingButton.js

@@ -48,6 +48,7 @@ export default class RatingButton extends Component<Props, State> {
       .fill(true)
       .map<Element<*>>((_, index) => (
         <div
+          key={index}
           role="button"
           tabIndex={0}
           onMouseEnter={() => {