Fail gracefully for node<4 in react-native-cli (const->var)

Summary:
`const` declarations were recently introduced into https://github.com/facebook/react-native/blob/master/react-native-cli/index.js, which runs before any transpilation so needs to be old-school. Replace them with `var`.

Fixes facebook#11603
Closes facebook#11615

Differential Revision: D5148448

Pulled By: shergin

fbshipit-source-id: ea23cdd26c0c1d0ac7f108aa9bf185abecfe399c

Author: rh389

react-native-cli/index.js

@@ -26,6 +26,9 @@
 // The only reason to modify this file is to add more warnings and
 // troubleshooting information for the `react-native init` command.
 //
+// To allow for graceful failure on older node versions, this file should
+// retain ES5 compatibility.
+//
 // Do not make breaking changes! We absolutely don't want to have to
 // tell people to update their global version of react-native-cli.
 //
@@ -266,9 +269,9 @@ function getInstallPackage(rnPackage) {
 }
 
 function run(root, projectName, options) {
-  const rnPackage = options.version; // e.g. '0.38' or '/path/to/archive.tgz'
-  const forceNpmClient = options.npm;
-  const yarnVersion = (!forceNpmClient) && getYarnVersionIfAvailable();
+  var rnPackage = options.version; // e.g. '0.38' or '/path/to/archive.tgz'
+  var forceNpmClient = options.npm;
+  var yarnVersion = (!forceNpmClient) && getYarnVersionIfAvailable();
   var installCommand;
   if (options.installCommand) {
     // In CI environments it can be useful to provide a custom command,