Bump minimal Node version to 4.7.0?

[gaearon]

Noticed this on master:

extract-text-webpack-plugin@2.0.0-rc.3: The engine "node" is incompatible with this module. Expected version ">=4.3.0 < 5.0.0 || >= 5.10".

[gaearon]

And even this:

webpack-dev-server@2.4.1: The engine "node" is incompatible with this module. Expected version ">=4.7".

[Timer]

I see no harm in this.

[gaearon]

Yea, just saying we need to update the docs and the CRA script once the minimal required version actually changes.

[ianschmitz]

Mind if I hop on this?

[Timer]

Sure, go ahead!

[gaearon]

Done in #1897.