Dont call npm before node is available
Dont call npm before node is available

Summary:
This fixes an error introduced in `0.18.0-rc`

`node` and `npm` isn't available if the developer is using `nvm` or `nodeenv`. XCode throws an error because of the call to `npm`. Simple move the line to after `node` and `npm` has been setup.
Closes https://github.com/facebook/react-native/pull/5156

Reviewed By: svcscm

Differential Revision: D2807589

Pulled By: androidtrunkagent

fb-gh-sync-id: 30c33145b2cb6f30ff67f6648153d5aa67fb74ed

eduvon0220/react-native,eduvon0220/react-native,eduvon0220/react-native,eduvon0220/react-native,eduvon0220/react-native,eduvon0220/react-native,eduvon0220/react-native,eduvon0220/react-native