[fixed] remove aria-modal attr to prevent browser bugs

There were a couple problems with the aria-modal attribute:

- it was not set on the correct dom element (in theory, it should be
set on the element having the dialog role, as stated in reactjs#654)
- it is buggy in Safari. The easy way to fix the problem is to remove the attribute
(again, as suggested in reactjs#654). While it's cool on paper, the attribute is currently
not necessary, since the `aria-hidden="true"` is set correctly on the
app element.

src/components/ModalPortal.js

@@ -334,7 +334,6 @@ export default class ModalPortal extends Component {
         style={{ ...overlayStyles, ...this.props.style.overlay }}
         onClick={this.handleOverlayOnClick}
         onMouseDown={this.handleOverlayOnMouseDown}
-        aria-modal="true"
       >
         <div
           ref={this.setContentRef}