deps: Upgrade graceful-fs dependency to the latest

[thefourtheye]

graceful-fs used to monkey-patch node's core fs module.
This has been fixed in the version 4.

Related: nodejs/node#2714

[ForbesLindesay]

Would you like to be added as a collaborator on this project, to help review and merge pull requests?

[thefourtheye]

@ForbesLindesay Sure. Thanks :-) Just in case if I have to ping you regarding this repo, what would be your preferred communication channel?

[ForbesLindesay]

Twitter is the best way to ping me if something's urgent or I've been ignoring something. My handle is:

@ForbesLindesay

[ForbesLindesay]

You're added. Please follow these rules:

1. never commit directly to master unless you're only editing documentation, or you're in the process of cutting a new release.
2. never merge your own pull request unless it's had no comments from any collaborators for at least 24 hours.
3. If you cut a new release, make sure you follow semver when updating the version number.

If you let me know your npm username I'll add you as an owner so you can release new versions :)