Changed MediaAccessibility Framework to weak reference

Framework was linked against binaries and since the framework is not
available before iOS7 that was causing the crash on launch.

Making the MediaAccessibility framework a weakly reference framework resolved the crash.

I checked this modification by building against https://github.com/mpropstbrightcove/VMNVideoPlayer-iOS-Sample and after the project successfully built testing the make the the app ran using iOS5, iOS6, and iOS7.