Fix issue pushing a modal twice

For some reason, when you open a screen in a modal StackNavigator, it
pushes twice.

I don't think this is the actual solution, but it does solve the problem
for me for now. I'll need more time to dig into this some more.

See this related issue for more: react-navigation#476 (comment)

src/routers/StackRouter.js

@@ -211,7 +211,7 @@ export default (
               routeToPush = initChildRoute;
             } else if (navigatedChildRoute !== initChildRoute) {
               // Push the route if the state has changed in response to this navigation
-              routeToPush = navigatedChildRoute;
+              routeToPush = initChildRoute;
             }
             if (routeToPush) {
               return StateUtils.push(state, {