fix(.nvmrc): change Node.js version from 4 to 6

[mgol]

.nvmrc was still pointing to Node.js 4 even though CLI will soon require 6.9 or
newer.

[johnpapa]

Can this do minors? or just majors?

[mgol]

It can do minors (and patches) as well but if you e.g. only have 4.7.3, 6.9.5 & 7.5.0 installed and .nvmrc would point to 4.6 then nvm use will not change the version and you'd be left with the previous one, 7.5.0 on my machine.

I know Angular uses a full version but I don't see a lot of gain in that while I see losses.

[mgol]

Is anything blocking this PR?

A discussion about whether to move to specify minors/patchers in .nvmrc can be led separately, this PR changes one major to another major and the current one is incorrect so it's a good change anyway. :)

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