[Bug][Jupyter Widget]

**Description**
Kepler.gl widgets in classic notebooks don't render inside [voila](https://github.com/QuantStack/voila) dashboards

**To Reproduce**
Steps to reproduce the behavior:
1. install voila
2. render the following code in a classic notebook:
```python
from keplergl import KeplerGl
KeplerGl(height=500)
```
3. Click on the "Voila" button in the notebook toolbar, see screenshot.

**Expected behavior**
A new page should open rendering the KeplerGl widget. But instead an empty page opens with only the text "User Guide: https://github.com/keplergl/kepler.gl/blob/master/docs/keplergl-jupyter/user-guide.md". Running the code in the notebook itself does produce the widget as expected, though.

**Screenshots**

**Environment:**
- Python version 3.7:
- Widget version: 0.1.0a
- OS version: macOS 10.14.6 Mojave