update Ruby to 2.5.8 (was 2.3.1) and nokogiri to 1.11.1

[SethTisue]

as per CVE-2020-26247

also: scala/scala-lang#1208

[SethTisue]

nokogiri-1.11.1-x86_64-linux requires ruby version < 3.1.dev, >= 2.5, which is incompatible with the current version, ruby 2.3.1p112 

@sjrd over at scala/scala-lang#1208 I was able to deal with this by bumping the Ruby version in .travis.yml, but in this repo we're on Drone with image: scalacenter/scala-rvm-jvm-coursier:2.0. What do you suggest we do?

• have someone at the Center update the Drone setup
• ditch Drone for Travis-CI or GitHub actions
  • context: the original Travis-to-Drone switch was made by Jorge at Add drone.yml and remove travis #685
  • I know Travis-CI but I don't know GitHub Actions, so if we go the Travis-CI route I'd be happy to just adapt the .travis.yml file we use over in the other repo, but if you want to go GitHub Actions instead, I'm not volunteering
• shrug and kick the problem down the road

?

[sjrd]

I'll put this up for discussion at the Center, and see if someone could take care of Drone or migrate to GitHub Actions. I don't think we should move anything to Travis at this point.

[vincenzobaz]

I am working on migrating to Github Actions in this PR

[vincenzobaz]

The ruby version is set in .github/workflows/build.yml, in theory changing Gemfile.lock and rebasing/merging on mastter shoud give you a green ci