There's currently an issue with running brew update on the Mac VMs, where you'll get an error along the lines of Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master.
Running git --work-tree=/usr/local --git-dir=/usr/local/.git clean -fd before brew update will fix this. The VMs will be updated later today with a fix, at which point the workaround is no longer needed, but it won't break anything either.