ExportTool fails

When I set `bokeh-export-tool` to true in `~/.dask/config.yaml` my bokeh pages no longer show. Instead I get the following error in the developer console

```
[bokeh] Failed to repull session Error: Model `ExportTool' does not exist. This could be due to a widget or a custom model not being registered before first usage.
```

Relevant code here: https://github.com/dask/distributed/blob/master/distributed/bokeh/export_tool.py
and here: https://github.com/dask/distributed/blob/master/distributed/bokeh/components.py#L108-L111

cc @bryevdv in case looking at the above quickly triggers something obvious