Bootstrap v4.0.0-beta.3

Hello! I’ve manually updated to beta.3 – is there any reason that Sage 9 is still using beta.2 or should it just work? Thanks in advance and all the best for 2018, Wolf-Dieter

There’s actually a super good reason: Beta 3 has only been out for a handfull of hours and it’s Saturday in the week between Christmas and New Years.

I’m sure a Pull Request would be well received, though!