Update deprecation warning s/6/7/

[ChALkeR]

See #61 (comment) — that message is not correct.

Only a runtime warning landed in Node.js 6.0, that warning will be turned into a throw in Node.js 7.0.

Refs: nodejs/node#6413, nodejs/node#5213, nodejs/node#5102.

[ChALkeR]

The initial graceful-fs deprecation message was fine, but it was mistakenly changed in #61.

[isaacs]

Aha! Sorry for my confusion. Updated deprecation warning. Thanks for the update.

[ChALkeR]

@isaacs Thanks!