[bugfix] Remove 'encoding' param in json.dumps to make it work for both

Python 2 and 3.

controlpanel.py

@@ -329,8 +329,10 @@ class ControlPanel(HasTraits):
     def save_to_file(self, file_name):
         """Save current settings into a configuration file."""
         with open(file_name, 'w') as f:
-            json.dump(self, f, indent=4, encoding='utf-8',
-                      default=self.json_codec.coder)
+            json.dump(self, f, indent=4, default=self.json_codec.coder)
+            # In Python 2.x, the 'encoding' param defaults to 'utf-8'.
+            # In Python 3.x, the 'encoding' param does not exist anylonger.
+            # Ergo, I removed "encoding='utf-8'" from json.dumps call.
 
     def _load_settings_fired(self):
         """Callback of the "load settings" button.