chore/config: update Node JS version requirement to ^8.0.0

[maidsafe-highfive]

r? @ustulation

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[hunterlester]

The issue with email app was associated with electron-forge, see: https://github.com/electron-userland/electron-forge/issues/249

It's not an issue if we just stick to using npm for email_app.

[bochaco]

I'm seeing this error when I launched the email app after packaing it with node v8.9.4:

Uncaught Error: The module '/tmp/.org.chromium.Chromium.lVf6KA'
was compiled against a different Node.js version using
NODE_MODULE_VERSION 57. This version of Node.js requires
NODE_MODULE_VERSION 53. Please try re-compiling or re-installing
the module (for instance, using `npm rebuild` or`npm install`).
    at process.module.(anonymous function) [as dlopen] (ELECTRON_ASAR.js:173:20)
    at Object.Module._extensions..node (module.js:598:18)
    at Object.module.(anonymous function) [as .node] (ELECTRON_ASAR.js:187:18)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)
    at Module.require (module.js:498:17)
    at require (internal/module.js:20:19)
    at bindings (/home/bochaco/work/bochaco-safe_examples/email_app/out/safe-mail-tutorial-linux-x64/resources/app.asar/node_modules/bindings/bindings.js:76:44)
    at Object.<anonymous> (/home/bochaco/work/bochaco-safe_examples/email_app/out/safe-mail-tutorial-linux-x64/resources/app.asar/node_modules/weak/lib/weak.js:7:35)

I tried with npm rebuild as the error suggests it but it doesn't solve it.

[bochaco]

Thanks @hunterlester for the help, the error I was seeing was due to the issue with yarn you mention in the README, it works fine with npm.