Update Layout children Type (gatsbyjs#64)

Having the error below with element type.
Failed prop type: Invalid prop `children` of type `array` supplied to `Layout`, expected a single ReactElement.

src/components/layout.js

@@ -43,7 +43,7 @@ const Layout = ({ children, data }) => (
 )
 
 Layout.propTypes = {
-  children: PropTypes.element.isRequired,
+  children: PropTypes.node.isRequired,
 }
 
 export default Layout