Git-ignore the Jekyll cache.

[lardieri]

Resolve #199

Jekyll 4.x generates a cache when running the test suite. Prevent accidental commits.

[lardieri]

@mattr- - Do you have the ability to merge and close this PR? I can't, because the CI checks are failing, so I don't get a Merge button.

To put this in context, every commit on every PR for this repo is currently failing in AppVeyor.

The RUBY_FOLDER_VER=23 test always fails with:

Bundler::Fetcher::CertificateFailureError Could not verify the SSL certificate for https://rubygems.org/

Also, other tests randomly fail on every pull request, even though they have nothing to do with the commits in question. This prevents anyone who is not a collaborator from merging changes, even with approval.

If you are not a project owner or maintainer, would you happen to know who is?

[lardieri]

@parkr and @benbalter for visibility. Until issue #204 gets resolved, no one can contribute to this project.