Replace React.PropTypes with usage of the prop-types package

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/components/views/elements/Dropdown.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2017 Vector Creations Ltd
+Copyright 2019 Michael Telatynski <7t3chguy@gmail.com>
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -58,7 +59,7 @@ class MenuOption extends React.Component {
 
 MenuOption.propTypes = {
     children: PropTypes.oneOfType([
-      PropTypes.arrayOf(React.PropTypes.node),
+      PropTypes.arrayOf(PropTypes.node),
       PropTypes.node,
     ]),
     highlighted: PropTypes.bool,