Bump graceful-fs dependency

[ChALkeR]

graceful-fs versions lower than v4 reevaluate native fs module source, which is not supported.
You should update your graceful-fs dependency to v4 or later.

Tracking issue: nodejs/node#5213.

[ChALkeR]

Hi. engine-munger is currently #34 in my list of modules that would likely to be broken when nodejs/node#6413 lands (which is anticipated to happen before v7.0).

Note that graceful-fs@1 and graceful-fs@4 are ok and would not be broken by this, but a require() on graceful-fs@2 or graceful-fs@3 would cause an error soon.

That is, runtime warning fs: re-evaluating native module sources is not supported. would be turned into a throw.

If you or your deps use graceful-fs@2 or graceful-fs@3 in runtime — you are affected.

This doesn't look to be fixed in the latest version, because file-resolver@1.0.2 depends on graceful-fs@2.0.3.

Tracking: nodejs/node#5213.

/cc @aredridel @grawk.

[ChALkeR]

Related: krakenjs/file-resolver#31.