Use pyyaml safe_load instead of load

Since load in unsafe (ref: https://msg.pyyaml.org/load),
switch the only occurrance of `yaml.load` that i could
find to safe_load.

closes #1085

core/admin/mailu/manage.py

@@ -177,7 +177,7 @@ def config_update(verbose=False, delete_objects=False):
     """sync configuration with data from YAML-formatted stdin"""
     import yaml
     import sys
-    new_config = yaml.load(sys.stdin)
+    new_config = yaml.safe_load(sys.stdin)
     # print new_config
     domains = new_config.get('domains', [])
     tracked_domains = set()