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 https://github.com/facebook/react-native/issues/11603
Closes https://github.com/facebook/react-native/pull/11615

Differential Revision: D5148448

Pulled By: shergin

fbshipit-source-id: ea23cdd26c0c1d0ac7f108aa9bf185abecfe399c