Use double quotes for JSX properties (facebook#225)

It helps to maintain consistency with React docs

Author: hnordt

template/README.md

@@ -99,7 +99,7 @@ import Button from './Button'; // Import a component from another file
 
 class DangerButton extends Component {
   render() {
-    return <Button color='red' />;
+    return <Button color="red" />;
   }
 }
 
@@ -139,7 +139,7 @@ import './Button.css'; // Tell Webpack that Button.js uses these styles
 class Button extends Component {
   render() {
     // You can use them as regular CSS styles
-    return <div className='Button' />;
+    return <div className="Button" />;
   }
 }
 ```