Remove logging from module stub generation

This is causing an error during deployment:

    Running "shell:generate_module_stubs" (shell) task
    Warning: stdout maxBuffer exceeded.Use --force to continue.

Relevant links from @lennym:

- https://github.com/sindresorhus/grunt-shell/blob/master/tasks/shell.js#L22
- http://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback

Nobody ever sees this non-error logging, so we should just remove it.
Merge pull request #438 from alphagov/remove-logging-module-generation

Remove logging from module stub generation