Accept a Node engine of version 6.X.X

[RaitoBezarius]

This should fix #6644 -- I ran the test suite with Node v6.0.0 and NPM v3.8.6, all the tests were passing.
Thanks in advance!

[zpao]

Yea, this should be fine. Thanks :)

[keyz]

When I run npm run build with node v6.0.0, the first 2 tasks (delete-build-modules and build-modules) warn about an outdated graceful-fs dep. The warnings don't show up when building on node v5.11.0.

I'm not sure which package has this dependency tho. Seems like it's gulp related. Should this be updated @zpao?

> grunt build

Running "delete-build-modules" task
(node:32080) fs: re-evaluating native module sources is not supported. If you are using the graceful-fs module, please update it to a more recent version.
[14:43:14] Using gulpfile ~/fb/react/gulpfile.js
[14:43:14] Starting 'react:clean'...
[14:43:14] Finished 'react:clean' after 34 ms

Running "build-modules" task
(node:32081) fs: re-evaluating native module sources is not supported. If you are using the graceful-fs module, please update it to a more recent version.
[14:43:14] Using gulpfile ~/fb/react/gulpfile.js
[14:43:14] Starting 'react:modules'...

[zpao]

Not much we can do if it's a dep we don't own. If you want to hunt it down and file an issue against whatever package is to blame, I encourage that :)

[keyz]

@zpao It's related to tooling issues like #6433. For instance, grunt 0.4.x requires glob that requires "graceful-fs": "~1.2.0".

However, I just diffed the build outputs on v6 and v5 and there's no difference. I guess we can ignore the warnings for now. 😀

[keyz]

Oh that being said, should node v5 and v6 be added to the travis build matrix? @zpao

[zpao]

Nah, we generally recommend using (and most/all of the team uses) LTS versions for sanity so I think we'll just do that on Travis to cut down on the build matrix. Build output should be the same regardless.