I've just got this message when I ran core. Have the miners started to mess with blocks already?
Look under your avatar in the "News" thing in bold (press F5 because it changes with each refresh and you may be seeing something else):
News: The warning which may be displayed by Bitcoin Core about unknown versions is related to BIP91, and can be safely ignored.
There should be a bigger sign or a sticky in the main section or something. I know I was scared as fuck this morning when I saw this, then I quickly realized it was probably due BIP91.