[react-native] Use trailing commas transform

Summary:
@public
Apparently trailing commas transform isn't exported by react-tools. We need to pull it out manually. This is not so clean but we're swtching to babel very shortly.

Test Plan:
* npm start
* write `foo(a,b,c,)` in some file
* request that file in the browser and make sure that trailing comma is gone

packager/transformer.js

@@ -16,10 +16,12 @@ var reactVisitors =
   require('react-tools/vendor/fbtransform/visitors').getAllVisitors();
 var staticTypeSyntax =
   require('jstransform/visitors/type-syntax').visitorList;
+var trailingCommaVisitors =
+  require('jstransform/visitors/es7-trailing-comma-visitors.js').visitorList;
+
 // Note that reactVisitors now handles ES6 classes, rest parameters, arrow
 // functions, template strings, and object short notation.
-var visitorList = reactVisitors;
-
+var visitorList = reactVisitors.concat(trailingCommaVisitors);
 
 function transform(srcTxt, filename) {
   var options = {