Run CircleCI tests on the latest version of Node

Summary:
Travis CI runs Node 4 (LTS), so make CircleCI run Node `<latest>` which is Node 6.
Closes https://github.com/facebook/react-native/pull/7542

Differential Revision: D3334397

fbshipit-source-id: 15a758011626fca4efdc4c1b36891a4d73c52e94

circle.yml

@@ -4,7 +4,7 @@ general:
       - gh-pages # list of branches to ignore
 machine:
   node:
-    version: 5.6.0
+    version: 6.2.0
   environment:
     PATH: "~/$CIRCLE_PROJECT_REPONAME/gradle-2.9/bin:/home/ubuntu/buck/bin:$PATH"
     TERM: "dumb"