Move requireNativeComponent call below the complete MapView definition

requireNativeComponent should be called after the wrapper component is fully defined, including its static properties.

docs/NativeComponentsIOS.md

@@ -87,8 +87,6 @@ class MapView extends React.Component {
   }
 }
 
-var RCTMap = requireNativeComponent('RCTMap', MapView);
-
 MapView.propTypes = {
   /**
    * When this property is set to `true` and a valid camera is associated
@@ -100,6 +98,8 @@ MapView.propTypes = {
   pitchEnabled: React.PropTypes.bool,
 };
 
+var RCTMap = requireNativeComponent('RCTMap', MapView);
+
 module.exports = MapView;
 ```