Update rdoc to remove an old jquery library

Summary

The version of rdoc we use 6.0.4 includes an old copy of jquery that has known security vulnerabilities. (jquery 1.6.4).

We should update to a newer rdoc version, whichs removes jquery entirely. Code diff: https://github.com/ruby/rdoc/compare/v6.0.4..v6.1.2. Also note that 6.1.2 is the default version of rdoc for our ruby version: https://stdgems.org/rdoc/#gem-version

Edited Jan 04, 2020 by silv