Use a separate babel config for the local-cli and the packager

Summary:
This separates the babel config of the local-cli and the packager from the one used by the transforms of the packager since it doesn't run in the same environment and the local-cli/packager doesn't require react specific transforms and runs in node 4 so we can also avoid some es2015 transforms that node already supports.

I had to move the code in cli.js so it can still run in node 0.12 that doesn't support `const` since it is no longer transformed.

**Test plan**
Run the local-cli on node 0.12 and there should be a message saying that it requires at least node 4.
Run the local-cli on node 4 and 5 and everything should work the same as before.

I was also hoping for some perf gains but there was nothing noticeable. I did benchmark the babel-register call and it stayed pretty much the same. As for runtime performance it can help if there are optimisations for es2015 features in node.
Closes https://github.com/facebook/react-native/pull/6155

Differential Revision: D3242754

Pulled By: davidaurelio

fb-gh-sync-id: 02880c841c10562d5f107e1c975d668e55cc619f
fbshipit-source-id: 02880c841c10562d5f107e1c975d668e55cc619f

local-cli/server/server.js

@@ -9,7 +9,6 @@
 'use strict';
 
 const chalk = require('chalk');
-const checkNodeVersion = require('./checkNodeVersion');
 const formatBanner = require('./formatBanner');
 const parseCommandLine = require('../util/parseCommandLine');
 const path = require('path');
@@ -88,8 +87,6 @@ function _server(argv, config, resolve, reject) {
     )
     : config.getAssetRoots();
 
-  checkNodeVersion();
-
   console.log(formatBanner(
     'Running packager on port ' + args.port + '.\n\n' +
     'Keep this packager running while developing on any JS projects. ' +