Fix tap exception in iOS

Hi Aaron,

I tried to use the tap event on the iPad running iOS 6.1.3 and I got the following exception:

TypeError: 'undefined' is not an object (evaluating 'e.changedTouches[0]')

// Thsi is how I set up the event handler.
$('.evp-volume').on("tap", function() { ... })

So I applied this patch, what do you think?

Thanks