Update ModalPortal.js (reactjs#228)

Is necessary to change contentHasFocus() to contentHasFocus: function() in order to avoid IE errors

Author: jsanchez556

lib/components/ModalPortal.js

@@ -169,7 +169,7 @@ var ModalPortal = module.exports = React.createClass({
     return !this.props.isOpen && !this.state.beforeClose;
   },
 
-  contentHasFocus() {
+  contentHasFocus: function() {
     return document.activeElement === this.refs.content || this.refs.content.contains(document.activeElement);
   },