Rename vercel configuration file from now.json to vercel.json (#738)

* Rename now.json to vercel.json

ZEIT changed their name to Vercel (https://vercel.com/blog/zeit-is-now-vercel).
The configuration file now.json also changed to vercel.json.

* Also Fix Zeit to Vercel in MAINTENANCE.md

Author: seisman

MAINTENANCE.md

@@ -80,13 +80,13 @@ tagged releases, and to PyPI for tagged releases only.
 
 ## Continuous Documentation
 
-We use the [Zeit Now for GitHub integration](https://zeit.co/github) to preview changes
+We use the [Vercel for GitHub](https://github.com/apps/vercel) App to preview changes
 made to our documentation website every time we make a commit in a pull request.
-The integration service has a configuration file `now.json`, with a list of options to
-change the default behaviour at https://zeit.co/docs/configuration.
-The actual script `package.json` is used by Zeit Now to install the necessary packages,
+The service has a configuration file `vercel.json`, with a list of options to
+change the default behaviour at https://vercel.com/docs/configuration.
+The actual script `package.json` is used by Vercel to install the necessary packages,
 build the documentation, copy the files to a 'public' folder and deploy that to the web,
-see https://zeit.co/docs/v2/build-step/?query=package.json#defining-a-build-script.
+see https://vercel.com/docs/build-step.
 
 ## Making a Release