Mark update-subrepos.sh as a zsh script

Stops some systems from running it through /bin/sh, which doesn't have a
pushd/popd builtin.
Make bin/update-subrepos.sh also update registered submodules

For some reason powerline's developer uses "develop" instead of "master"
for the master branch, so we have to special case that in the `git
submodule foreach` command.