Change README install instructions to 3.1.0

Now that we have both the python and javascript packages to keep compatible we're better off just specifying all of the versions explicitly on each release.

Author: jonmmease

README.md

@@ -29,7 +29,7 @@ Built on top of [plotly.js](https://github.com/plotly/plotly.js), `plotly.py` is
 ## Installation of plotly.py Version 3
 To install plotly.py and enable Jupyter or Jupyter Lab support, run:
 ```
-pip install "plotly>=3.0"
+pip install plotly==3.1.0
 pip install "notebook>=5.3" "ipywidgets>=7.2"  # only necessary for Jupyter Notebook environments
 ```