fix propType of children

Author: nkzawa

lib/frame.js

@@ -16,7 +16,7 @@ export default class Frame extends React.Component {
 }
 
 Frame.propTypes = {
-  children: PropTypes.element.isRequired,
+  children: PropTypes.arrayOf(PropTypes.element).isRequired,
   component: PropTypes.string,
   delay: PropTypes.number,
   onRender: PropTypes.func

lib/keyframes.js

@@ -70,7 +70,7 @@ export default class Keyframes extends React.Component {
 }
 
 Keyframes.propTypes = {
-  children: PropTypes.element.isRequired,
+  children: PropTypes.arrayOf(PropTypes.element).isRequired,
   component: PropTypes.string,
   onStart: PropTypes.func,
   onEnd: PropTypes.func