bring back ruby 2.4 since that broke rubocop builds because of some d…

[grosser]

…ependency foobar

see d191db7#commitcomment-44432975

Installing parallel 1.20.0
Gem::RuntimeRequirementNotMetError: parallel requires Ruby version >= 2.5. The
current ruby version is 2.4.10.364.
An error occurred while installing parallel (1.20.0), and Bundler
cannot continue.
Make sure that `gem install parallel -v '1.20.0' --source
'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  rubocop-performance was resolved to 1.9.0, which depends on
    rubocop was resolved to 1.3.1, which depends on
      parallel