ValueError: Can't clean for JSON for intake.catalog.local.LocalCatalogEntry

Running into an error outputting an intake.catalog.local.LocalCatalogEntry in a jupyter notebook. print(entry) works, but display(entry) a `ValueError: Can't clean for JSON`

pinging @jhamman and @martindurant for help sorting this one out. I think it's likely a simple fix.

```python
import intake
import intake_stac
print(intake.__version__) #0.5.3
print(intake_stac.__version__) #0.2.1

cat = open_stac_catalog('https://storage.googleapis.com/pdd-stac/disasters/catalog.json')
list(cat)
entry = cat['Houston-East-20170831-103f-100d-0f4f-RGB']
type(entry) #intake.catalog.local.LocalCatalogEntry
print(entry)
"""
name: Houston-East-20170831-103f-100d-0f4f-RGB
container: catalog
plugin: ['stac_item']
description:
direct_access: True
user_parameters: []
metadata:
args:
stac_obj: Houston-East-20170831-103f-100d-0f4f-RGB
"""
display(entry)
```

```pytb
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
/srv/conda/envs/notebook/lib/python3.7/site-packages/IPython/core/formatters.py in __call__(self, obj)
916 method = get_real_method(obj, self.print_method)
917 if method is not None:
--> 918 method()
919 return True
920

/srv/conda/envs/notebook/lib/python3.7/site-packages/intake/catalog/entry.py in _ipython_display_(self)
113 }, metadata={
114 'application/json': {'root': contents["name"]}
--> 115 }, raw=True)
116
117 def __getattr__(self, attr):

/srv/conda/envs/notebook/lib/python3.7/site-packages/IPython/core/display.py in display(include, exclude, metadata, transient, display_id, *objs, **kwargs)
309 for obj in objs:
310 if raw:
--> 311 publish_display_data(data=obj, metadata=metadata, **kwargs)
312 else:
313 format_dict, md_dict = format(obj, include=include, exclude=exclude)

/srv/conda/envs/notebook/lib/python3.7/site-packages/IPython/core/display.py in publish_display_data(data, metadata, source, transient, **kwargs)
120 data=data,
121 metadata=metadata,
--> 122 **kwargs
123 )
124

/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel/zmqshell.py in publish(self, data, metadata, source, transient, update)
127 # hooks before potentially sending.
128 msg = self.session.msg(
--> 129 msg_type, json_clean(content),
130 parent=self.parent_header
131 )

/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel/jsonutil.py in json_clean(obj)
189 out = {}
190 for k,v in iteritems(obj):
--> 191 out[unicode_type(k)] = json_clean(v)
192 return out
193 if isinstance(obj, datetime):

/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel/jsonutil.py in json_clean(obj)
189 out = {}
190 for k,v in iteritems(obj):
--> 191 out[unicode_type(k)] = json_clean(v)
192 return out
193 if isinstance(obj, datetime):

/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel/jsonutil.py in json_clean(obj)
189 out = {}
190 for k,v in iteritems(obj):
--> 191 out[unicode_type(k)] = json_clean(v)
192 return out
193 if isinstance(obj, datetime):

/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel/jsonutil.py in json_clean(obj)
189 out = {}
190 for k,v in iteritems(obj):
--> 191 out[unicode_type(k)] = json_clean(v)
192 return out
193 if isinstance(obj, datetime):

/srv/conda/envs/notebook/lib/python3.7/site-packages/ipykernel/jsonutil.py in json_clean(obj)
195
196 # we don't understand it, it's probably an unserializable object
--> 197 raise ValueError("Can't clean for JSON: %r" % obj)

ValueError: Can't clean for JSON: Houston-East-20170831-103f-100d-0f4f-RGB
```