[fixed] Fix this scope (#296)

* [fixed] Make use of es6 modules

* [fixed] Fix `this` scope

Author: hiroppy

examples/basic/app.js

@@ -1,6 +1,6 @@
-var React = require('react');
-var ReactDOM = require('react-dom');
-var Modal = require('../../lib/index');
+import React from 'react';
+import ReactDOM from 'react-dom';
+import Modal from '../../lib/index';
 
 var appElement = document.getElementById('example');
 

lib/components/ModalPortal.js

@@ -137,7 +137,7 @@ export default class ModalPortal extends Component {
     });
   }
 
-  closeWithoutTimeout () {
+  closeWithoutTimeout = () => {
     this.setState({
       beforeClose: false,
       isOpen: false,