[Cake] Cobalt is the new master

[Toke Høiland-Jørgensen]

Hey everyone

Since the master branch of the sch_cake repo has been broken for quite
some time, and all the upstreaming work has gone into the cobalt branch,
it's been clear for a while that we should stop having master as the
default.

Triggered by the latest report of the master branch not working, I just
force pushed the cobalt branch into master on Github. This is just a
head's up in case anyone tries to pull from master. What used to be the
master branch now lives in the old-master branch.

The cobalt branch is still around, but is now identical to master. Any
future updates will probably only be in the master branch.

-Toke