Match native iphone/ipad index.html urls also

This came about from trying to debug the native iphone application. The native application was trying to load '/r/checkout/iphone//index.html' which resulted in a non-match and hence would pull from proxy to production's version. This fix matches that url and then loads the local development box's iphone/index.html file correctly. That way any changes made in native_checkout.js for example can be tested.