added branch to ignore for CI tests

Summary:
every push to gh-pages renders an error in tests https://circleci.com/gh/facebook/react-native/995?utm_campaign=build-failed&utm_medium=email&utm_source=notification
Closes #5109

Reviewed By: svcscm

Differential Revision: D2802873

Pulled By: androidtrunkagent

fb-gh-sync-id: 8043f47ccfce322ae26ac5e85604bcc2c79d9cb4

Author: bestander

circle.yml

@@ -1,3 +1,7 @@
+general:
+  branches:
+    ignore:
+      - gh-pages # list of branches to ignore
 machine:
   node:
     version: 5.1.0