fix stderr maxBuffer error on npm pack

[psnyder]

This fixes the following error, which occurs on the npm pack call because exec was used instead of spawn. Updated to use spawn and all is well.

~/.nvm/versions/node/v8.12.0/lib/node_modules/npm-bundle/bin/cli.js:11
    throw error
    ^

Error: stderr maxBuffer exceeded
    at Socket.onChildStderr (child_process.js:347:14)
    at emitOne (events.js:116:13)
    at Socket.emit (events.js:211:7)
    at addChunk (_stream_readable.js:263:12)
    at readableAddChunk (_stream_readable.js:246:13)
    at Socket.Readable.push (_stream_readable.js:208:10)
    at Pipe.onread (net.js:601:20)