Use a more typical optional default value for optional import in watcher.py.

Thanks to @nsoranzo for the advice - https://github.com/galaxyproject/galaxy/pull/1448#discussion_r49210125.

lib/galaxy/tools/toolbox/watcher.py

@@ -11,7 +11,7 @@ try:
 except ImportError:
     Observer = None
     FileSystemEventHandler = object
-    PollingObserver = object
+    PollingObserver = None
     can_watch = False
 
 log = logging.getLogger( __name__ )