fix us-west-1 precise64 ami

Change made in other tempates, doing for this template too. At some
point there was a copy/paste error and the eu-west-1 was used for
us-west-1. See 2cba8413

cloudformation_templates/edx-reference-architecture.json

@@ -235,7 +235,7 @@
       },
       "us-west-1":{
         "32":"ami-fc002cb9",
-        "64":"ami-ce7b6fba"
+        "64":"ami-fe002cbb"
       },
       "us-west-2":{
         "32":"ami-0ef96e3e",