Update gulp to v4.0

[enzy]

When it's published

https://github.com/gulpjs/gulp/blob/4.0/CHANGELOG.md
https://github.com/gulpjs/gulp/tree/4.0
gulpjs/gulp#861

[enzy]

As gracefull-fs id deprecated, but gulp depends on it:

npm WARN deprecated graceful-fs@1.2.3: graceful-fs v3.0.0 and before will fail on node releases >= v7.0. Please update to graceful-fs@^4.0.0 as soon as possible. Use 'npm ls graceful-fs' to find it in the tree.

+-- gulp@3.9.1
| `-- vinyl-fs@0.3.14
|   +-- glob-watcher@0.0.6
|   | `-- gaze@0.5.2
|   |   `-- globule@0.1.0
|   |     `-- glob@3.1.21
|   |       `-- graceful-fs@1.2.3

[enzy]

Implemented in master branch