DisplayFormatter.formatters should not be configurable

it is a dict of instances, which are not allowed in config

IPython/core/formatters.py

@@ -44,7 +44,7 b' class DisplayFormatter(Configurable):'
 
     # A dict of formatter whose keys are format types (MIME types) and whose
     # values are subclasses of BaseFormatter.
-    formatters = Dict(config=True)
+    formatters = Dict()
     def _formatters_default(self):
         """Activate the default formatters."""
         formatter_classes = [