Fix broken CI

[ysk24ok]

But this is just a hot fix and it'll break again when the new version of sass-embedded is released, which depends on the newer version of rubygems. If the new version starts to depend on rubygems >= 3.4 which comes with bundler 2.4, this would be a problem as appraisal can't work with bundler 2.4.

There are several alternative solutions:

• Use jekyll-sass-converter < 3.0. But this is not ideal as we can't keep up with its new versions.
• Remove appraisal from dependencies of this plugin. As the latest 2.4.1 was released in July 2021, I don't think it's sufficiently maintained. I'd need to look for other libraries to enable testing with several versions of Jekyll. But testing with the latest Jekyll might be sufficient.