Update Jekyll::HighlightBlock to Jekyll::Tags::HighlightBlock
Update Jekyll::HighlightBlock to Jekyll::Tags::HighlightBlock

This was renamed in Jekyll quite a while ago.  Update our code to match.

Signed-off-by: Kevin Locke <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinlocke.name>

kevinoid/jekyll-highlight-file