fix issue with time/duration message fields (without std_msgs prefix)…

… when used as array (ros/ros_comm#252)

src/genpy/message.py

@@ -523,6 +523,10 @@ def _get_message_or_service_class(type_str, message_type, reload_on_error=False)
     :returns: Message/Service  for message/service type or None, ``class``
     :raises: :exc:`ValueError` If message_type is invalidly specified
     """
+    if message_type == 'time':
+        return Time
+    if message_type == 'duration':
+        return Duration
     ## parse package and local type name for import
     package, base_type = genmsg.package_resource_name(message_type)
     if not package: