Check that unions exists before promisifying.

Author: GabeIsman

lib/acl.js

@@ -65,7 +65,9 @@ var Acl = function (backend, logger, options){
   backend.getAsync = bluebird.promisify(backend.get);
   backend.cleanAsync = bluebird.promisify(backend.clean);
   backend.unionAsync = bluebird.promisify(backend.union);
-  backend.unionsAsync = bluebird.promisify(backend.unions);
+  if (backend.unions) {
+    backend.unionsAsync = bluebird.promisify(backend.unions);
+  }
 };
 
 /**
@@ -432,7 +434,7 @@ Acl.prototype.allowedPermissions = function(userId, resources, cb){
     .params('string|number', 'string|array')
     .end();
 
-  if (this.backend.unions) {
+  if (this.backend.unionsAsync) {
     return this.optimizedAllowedPermissions(userId, resources, cb);
   }