Change plotly.js to version 2.18.1 (#4)

- Change plot;y.js to version 2.18.1 (2abd1c752e97301b45b9fa5d218fb1e4de76a8a8)

src/streamlit_app.py

@@ -242,7 +242,7 @@ points_json = json.dumps(points)
 
 ## build up plot and image container html 
 plot_html = f"""
-<script src="https://cdn.plot.ly/plotly-3.0.1.min.js"></script>
+<script src="https://cdn.plot.ly/plotly-2.18.1.min.js"></script>
 <style>
   html, body {{
     margin:0; padding:0; height:100%;