Use legacy methods to disable UIView animations for iOS 6 in keepNotAnimated:

iOS 6 is probably not that relevant nowdays, but wouldn't it be better to provide consistent behavior when the `performWithoutAnimation:` method is unavailable?

As per documentation for `setAnimationsEnabled:`
>This method affects only those animations that are submitted after it is called. If you call this method while existing animations are running, those animations continue running until they reach their natural end point.

So it looks like the behavior would be essentially the same as with the block-based `performWithoutAnimation:` call. 

It also seems like the whole `keepNotAnimated:` method could be enabled for any iOS version with this change, but I'm not sure if there were other reasons to wrap it in the `__IPHONE_OS_VERSION_MAX_ALLOWED` check.