Export the hmr plugin directly

Summary: This will allow to configure the HMR plugin directly from `.babelrc` again, instead of having to create a bridge file: https://github.com/rafeca/metro-sample-app/blob/master/metro-babel7-plugin-react-transform.js#L3

Reviewed By: davidaurelio

Differential Revision: D7707314

fbshipit-source-id: 4c5612e1e5d27874807f2dce50d99ec0f6354bbc

babel-preset/configs/hmr.js

@@ -24,7 +24,7 @@ module.exports = function(options, filename) {
   return {
     plugins: [
       [
-        require('metro-babel7-plugin-react-transform').default,
+        require('metro-babel7-plugin-react-transform'),
         {
           transforms: [{
             transform: transform,