switch node 8 image to official one (#23425)

Summary:
node 8 image has yarn too. Also the official one has the latest yarn.
Pull Request resolved: https://github.com/facebook/react-native/pull/23425

Differential Revision: D14065105

Pulled By: cpojer

fbshipit-source-id: 4eb3347fc191d928091e984fe3e5887861ec8aa9

.circleci/config.yml

@@ -276,9 +276,7 @@ defaults: &defaults
 js_defaults: &js_defaults
   <<: *defaults
   docker:
-    - image: circleci/node:8
-  environment:
-    - PATH: "/opt/yarn/yarn-v1.5.1/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+    - image: node:8
 
 android_defaults: &android_defaults
   <<: *defaults