Fix ie9 loading

Apparently, ie9 can't be trusted to always fire the `complete` event. I have a case where it's not when the cache is empty, but does when the cache is primed. I don't know exactly what ie9's behavior is supposed to be. It looks like there is a fair amount of confusion on the point:

* http://stackoverflow.com/questions/1929742/can-script-readystate-be-trusted-to-detect-the-end-of-dynamic-script-loading
* http://stackoverflow.com/questions/6946631/dynamically-creating-script-readystate-never-complete
* http://msdn.microsoft.com/en-us/library/ie/ms534359(v=vs.85).aspx

In any event, this fixes the problem.