Update notebook version requirement in CONTRIBUTING.md too.

CONTRIBUTING.md

@@ -45,7 +45,7 @@ You can also use the installer from the [Node.js](https://nodejs.org) website.
 
 ## Installing JupyterLab
 
-JupyterLab requires Jupyter Notebook version 4.2 or later.
+JupyterLab requires Jupyter Notebook version 4.3 or later.
 
 If you use ``conda``, you can install it as:
 
@@ -148,7 +148,7 @@ build tool.  The npm package source files are in the `packages/` subdirectory.
 **Prerequisites**
 
 - [node](http://nodejs.org/) (preferably version 5 or later)
-- Jupyter notebook server version 4.2 or later (to run examples)
+- Jupyter notebook server version 4.3 or later (to run examples)
 
 ```bash
 npm install --save jupyterlab
@@ -174,7 +174,7 @@ npm run build:packages
 
 The Jupyter server extension source files are in the `jupyterlab/`
 subdirectory. To use this extension, make sure the Jupyter notebook server
-version 4.2 or later is installed.
+version 4.3 or later is installed.
 
 ### Build the JupyterLab server extension