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

packager/react-native-xcode.sh

@@ -29,9 +29,6 @@ cd ..
 set -x
 DEST=$CONFIGURATION_BUILD_DIR/$UNLOCALIZED_RESOURCES_FOLDER_PATH
 
-# npm global install path may be a non-standard location
-PATH="$(npm prefix -g)/bin:$PATH"
-
 # Define NVM_DIR and source the nvm.sh setup script
 [ -z "$NVM_DIR" ] && export NVM_DIR="$HOME/.nvm"
 
@@ -46,6 +43,9 @@ if [[ -x "$HOME/.nodenv/bin/nodenv" ]]; then
   eval "$($HOME/.nodenv/bin/nodenv init -)"
 fi
 
+# npm global install path may be a non-standard location
+PATH="$(npm prefix -g)/bin:$PATH"
+
 react-native bundle \
   --entry-file index.ios.js \
   --platform ios \