Rename vercel configuration file from now.json to vercel.json

[seisman]

Description of proposed changes

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

Fixes #447

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[weiji14]

There's some references to 'Zeit Now' at https://github.com/GenericMappingTools/pygmt/blob/master/MAINTENANCE.md#continuous-documentation which we should rename to "Vercel".

[weiji14]

Nice! Thanks @seisman!