fix bug with nextFrameID

Hello,

I had a problem with Sequence using the callback beforeNextFrameAnimatesIn. The problem occurs when using the pagination links : the nextFrameID variable don't have the good value (it has the value of currentFrameID). However, when using  next and prev buttons, nextFrameID has the right value. 

So i looked at the code and noticed self.nextFrameID is set in the next/prev buttons click handlers before the callback is called. But it isn't in the pagination handler. So i added it and it seems to work now

This is a one line change commit, so you might figure out quick if it's correct or not :)

Regards,
Fred