Switched to format for proxy msg

To handle point 1. in #110 (comment)

Author: Joe Yu

python/tk_desktop/site_communication.py

@@ -74,7 +74,7 @@ def _proxy_log(self, level, msg, args):
         """
         try:
             # Format first so logger.exception can use it
-            msg = "[PROXY] %s" % msg
+            msg = "[PROXY] {0}".format(msg)
             logger.log(level, msg, *args)
         except Exception:
             message = (