Fix incompatibility with Jekyll 4.2

[fauno]

@source_dir is new to jekyll 4.2 and made the build fail when trying to debug output in Jekyll::DataReader#read_data_file. I also found that the initialize code is the same so I changed it to use super, not sure if you wanted to keep it that way, I can rollback those changes.

tests pass but 4 are failing because rouge is adding a language class to the output and is different from the fixture. I can fix that too.

Closes #39

[ashmaroli]

Thank you for submitting this PR, @fauno.
Regarding the changes, I prefer having a bare super call instead of super(site).. and RuboCop has detected an offense as well.

---

Note to self: Migrate CI from Travis to GH Actions before merging this..

[fauno]

ok! rubocop isn't telling me that now though, but it's true i forgot to run it :)

[ashmaroli]

Thanks @fauno