Expose NavigatorIOS::replaceAtIndex

Summary:
The React Native docs list `replaceAtIndex` as a public method of the **NavigatorIOS** component but it is never exposed.
Closes #10577

Differential Revision: D4168180

Pulled By: hramos

fbshipit-source-id: 6d91d9de8404bb229930d804f8b01b324ec16f24

Libraries/Components/Navigation/NavigatorIOS.ios.js

@@ -506,6 +506,7 @@ var NavigatorIOS = React.createClass({
       pop: this.pop,
       popN: this.popN,
       replace: this.replace,
+      replaceAtIndex: this.replaceAtIndex,
       replacePrevious: this.replacePrevious,
       replacePreviousAndPop: this.replacePreviousAndPop,
       resetTo: this.resetTo,