Bump to Node 8

Summary:
Moving to Node 8 in React Native will allow us to be consistent with Facebook, e.g. trailing commas.

As of April 30, Node 4.x is end of life, and Node 6.x entered maintenance mode (see https://medium.com/the-node-js-collection/april-2018-release-updates-from-the-node-js-project-71687e1f7742).

This will require our docs to be updated to require Node 8 as a minimum. This should be done as part of the following PR: facebook/react-native-website#355

Circle CI already runs all JS tests using Node 8, and it has an extra compatibility check that uses Node 6. This workflow is dropped as we no longer need to check for failures when Node 6 is used.

See facebook/react-native#19226

[GENERAL] [BREAKING] [Node] - Bump minimum req. Node version to 8
Closes facebook/react-native#19230

Differential Revision: D7979478

Pulled By: hramos

fbshipit-source-id: c91ec118f0a59c4daae5fcdbfc822ccead591304

server/checkNodeVersion.js

@@ -14,7 +14,7 @@ var formatBanner = require('metro-core/src/formatBanner');
 var semver = require('semver');
 
 module.exports = function() {
-  if (!semver.satisfies(process.version, '>=4')) {
+  if (!semver.satisfies(process.version, '>=8')) {
     var engine = semver.satisfies(process.version, '<1') ? 'Node' : 'io.js';
 
     var message =
@@ -24,15 +24,15 @@ module.exports = function() {
       process.version +
       '.\n' +
       '\n' +
-      'React Native runs on Node 4.0 or newer. There are several ways to ' +
+      'React Native runs on Node 8.0 or newer. There are several ways to ' +
       'upgrade Node.js depending on your preference.\n' +
       '\n' +
       'nvm:       nvm install node && nvm alias default node\n' +
       'Homebrew:  brew unlink iojs; brew install node\n' +
       'Installer: download the Mac .pkg from https://nodejs.org/\n' +
       '\n' +
       'About Node.js:   https://nodejs.org\n' +
-      'Follow along at: https://github.com/facebook/react-native/issues/2545';
+      'Follow along at: https://github.com/facebook/react-native/issues/19226';
     console.log(
       formatBanner(message, {
         chalkFunction: chalk.green,