Update README to version 4.2.0

Author: jonmmease

README.md

@@ -78,12 +78,12 @@ Built on top of [plotly.js](https://github.com/plotly/plotly.js), `plotly.py` is
 
 plotly.py may be installed using pip...
 ```
-pip install plotly==4.1.1
+pip install plotly==4.2.0
 ```
 
 or conda.
 ```
-conda install -c plotly plotly==4.1.1
+conda install -c plotly plotly=4.2.0
 ```
 
 ### Jupyter Notebook Support
@@ -126,10 +126,10 @@ set NODE_OPTIONS=--max-old-space-size=4096
 jupyter labextension install @jupyter-widgets/[email protected] --no-build
 
 # FigureWidget support
-jupyter labextension install plotlywidget@1.1.1 --no-build
+jupyter labextension install plotlywidget@1.2.0 --no-build
 
 # and jupyterlab renderer support
-jupyter labextension install jupyterlab-plotly@1.1.2 --no-build
+jupyter labextension install jupyterlab-plotly@1.2.0 --no-build
 
 # JupyterLab chart editor support (optional)
 jupyter labextension install [email protected] --no-build