Use `--json` flag for internal calls to npm

[charlespierce]

With npm versions 5.0.0 and above, you can pass --json to npm install and get machine-readable output from the call. This could be useful to provide better error messages when things go wrong (the JSON for an error includes the summary and details that are printed to stderr). It could also be useful for when things are successful, as it includes a manifest of all packages that changed.