Infer platform from extension

Summary: public
Aparently this used to work on 0.11, lets fix it :)

Reviewed By: foghina

Differential Revision: D2557719

fb-gh-sync-id: dcbca077431c1356c89dfc55b71eecff64c7ad3d

local-cli/bundle/bundleCommandLineArgs.js

@@ -18,7 +18,6 @@ module.exports = [
     command: 'platform',
     description: 'Either "ios" or "android"',
     type: 'string',
-    required: true,
   }, {
     command: 'transformer',
     description: 'Specify a custom transformer to be used (absolute path)',