update: make `git pull` update origin/master

Specify the refspec 'refs/heads/master:refs/remotes/origin/master' so
that the remote-tracking branch origin/master gets update during `brew
update` (git-pull updates remote-tracking branches when invoked
like `git pull` or `git pull origin`, but not `git pull origin master`).

Signed-off-by: Jack Nagel <jacknagel@gmail.com>