rudy_dev on Debian 9

[Egbert]

Ouch... Trying out the Octopress on Debian 9...

Got this error:

Fetching jekyll-paginate 1.1.0
Installing jekyll-paginate 1.1.0
Fetching jekyll-sass-converter 1.5.1
Installing jekyll-sass-converter 1.5.1
Fetching ruby_dep 1.5.0
Installing ruby_dep 1.5.0
Gem::InstallError: ruby_dep requires Ruby version >= 2.2.5, ~> 2.2.
An error occurred while installing ruby_dep (1.5.0), and Bundler
cannot continue.
Make sure that `gem install ruby_dep -v '1.5.0'` succeeds before bundling.


In Gemfile:
  octopress-date-format was resolved to 2.0.2, which depends on
    octopress-hooks was resolved to 2.6.2, which depends on
      jekyll was resolved to 2.5.3, which depends on
        jekyll-watch was resolved to 1.5.1, which depends on
          listen was resolved to 3.1.5, which depends on
            ruby_dep