Add `ast: true` to transform options for jest

Summary:
This should (at least partially) fix testing for OSS as per f8d6b97140 (r28647044)

Might be related to not locking down the Babel version, will do that in a different commit.

Reviewed By: mjesun

Differential Revision: D7685279

fbshipit-source-id: f00febfad2cec31b7e11996ee8efa2c96b44477b

jest/preprocessor.js

@@ -46,6 +46,7 @@ module.exports = {
       localPath: file,
       options: {
         assetDataPlugins: [],
+        ast: true, // needed for open source (?) https://github.com/facebook/react-native/commit/f8d6b97140cffe8d18b2558f94570c8d1b410d5c#r28647044
         dev: true,
         inlineRequires: true,
         minify: false,