Update supervisord

This has fixed an issue of not being able to use the interactive
supervisorctl command.

Note that this just updates to a commit roughly two weeks ago, because
the subsequent commit - 5937dd8c375a876275c413bbffc4ad25e2dc060d - fails
to build on nix due to test errors.
Make node available in policycompass_frontend nix environment
Start frontend server with node instead of npm

This closes policycompass/policycompass-frontend#8.
Fix policycompass-services supervisord start command

This makes the process not fork and thus allows it to be stopped via
supervisord.

Note that the builtin django webserver should be replaced with gunicorn
or uwsgi in production at some point.