Update gitlab_kramdown to v0.17.0

This brings in support for clickable PlantUML images (gitlab-org/ruby/gems/gitlab_kramdown!64 (merged)).

Example page to test: http://sh-update-plantuml-clickable.178.62.207.141.nip.io/ee/administration/reference_architectures/5k_users.html

Existing images like the Creative Commons logo in http://sh-update-plantuml-clickable.178.62.207.141.nip.io/ still have this icon, but should they?