This runs 8 `git submodule update` processes in parallel, speeding up
bootstrap from 2 minutes to 30 seconds for me. (Obviously this is
dependent on bandwidth.)
This makes it so we don't have to redownload all the grammars every time
we build the grammars gem. It will also let us verify that grammars.yml
is accurate in the future by checking it against the submodules on disk.
script/bootstrap now updates the submodules.