Update what's new in 2.14 to also include 2.15

Earlier this evening people seemed a bit surprised that there
isn't a 2.15 talk in the schedule.

I'll update my slides to add a brief overview of the new stuff
in 2.15, based on the content of Dave's talk from the 2017 summit.

Change-Id: I0b1b5f8402f12073dd805b9d412faf2633abfed1