Add missing JSON schema for 'refs' and 'branch_name'

Fixes #325

endpoints/api/trigger.py

@@ -400,6 +400,14 @@ class ActivateBuildTrigger(RepositoryParamResource):
         'commit_sha': {
           'type': 'string',
           'description': '(Custom Only) If specified, the ref/SHA1 used to checkout a git repository.'
+        },
+        'refs': {
+          'type': 'array',
+          'description': '(SCM Only) If specified, the refs to build.'
+        },
+        'branch_name': {
+          'type': 'string',
+          'description': '(SCM Only) If specified, the branch to build.'
         }
       },
       'additionalProperties': False