On running jekyll serve I was getting this error:

jekyll serve
Configuration file: /Users/ben/git/benhamilton.github.io/_config.yml
       Deprecation: The 'gems' configuration option has been renamed to 'plugins'. Please update your config file accordingly.

To fix it, the _config.yml file needed the following:

gems:
  - jekyll-sitemap # Create a sitemap using the official Jekyll sitemap gem
  - jekyll-feed # Create an Atom feed using the official Jekyll feed gem

changes to:

plugins:
  - jekyll-sitemap # Create a sitemap using the official Jekyll sitemap gem
  - jekyll-feed # Create an Atom feed using the official Jekyll feed gem

Now when running jekyll serve I no longer get the error.