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 That would be awesome.
I would love to do that. Thanks :-)

[ForbesLindesay]

Cool, 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 :)

[thefourtheye]

@ForbesLindesay Thanks for the onboarding guidelines :-) My npm handle is thefourtheye.