React does not support node 7.x for development

[Shastel]

What is the current behavior?
AssertionError: Current node version is not supported for development, expected "7.0.0" to satisfy "4.x || 5.x || 6.x".

Steps to reproduce

1. Install node 7.x
2. Clone React repo
3. run npm i

What is the expected behavior?
React support node v7 for development

[gaearon]

Thanks for the fix.