Support for Jekyll 4.0

[thejimnicholson]

I've modified the Gemfile and .gemspec to allow this to work with Jekyll 4.0. I've tested it as much as I can with a new site I'm building. On a fresh jekyll site, I had to modify _config.yml and the Gemfile to include jekyll-paginator to avoid some deprecation warnings, but other than that, it seems to work.

[dchaley]

Hello! I found this since I was trying to work with a new jekyll setup using version 4.

I can confirm that this PR makes it work, although yes I needed to include jekyll-paginate (not paginator) to work around the deprecation warning. It's still telling me this on jekyll serve :

Pagination: Pagination is enabled, but I couldn't find an index.html page to use as the pagination template. Skipping pagination.

[goplusgo]

Hope this PR gets merged ASAP.

[davidtmiller]

Thanks for going through the process of testing this out! I've tested things out on my end and I'm not getting any errors so I think we are good to go with this change. Thanks again!