IPython slides: fragments are now in a linear flow and include next "-" cells.

This PR solves, in general, the issue raise in #4356, but in a more simple way... using the new data-fragment-index attribute from Reveal and some little logic to mark the next cells as fragments continuations.
In particular, this PR address:
* update reveal.js CDN
* fragments are now more than a cell-level property, because now you can include more than one cell in a fragment.
* "-" (do nothing) cells are now included with the previous fragment (in the case there is one).

This is what I pretend to do when I discussed this issue with @pelson (but data-fragment-index attribute was buggy at that time). He had/has valid points in the discussion, and... I still think the previous design was not a bug, instead a choice, but I also recognize what the majority of the people is asking for, so here you have it :wink: but in a more compatible way with the current code design/structure :stuck_out_tongue_winking_eye: 

Pinging @Carreau, @dhirschfeld and @astrofrog, because they were pinged in the last discussion and probably they are interested...

OK, that's all for now... cheers.