FIX: pins down highlight.js to the last commit of the v9 tree (PR #9921)

The 10+ version has actually a bug with bash. We will wait a little bit before upgrading to 10, we should also make sure any of the breaking changes listed here https://github.com/highlightjs/highlight.js/blob/master/VERSION_10_BREAKING_CHANGES.md are not impacting us.

GitHub