Ruby 2.6 offers better performance and memory usage:
https://www.rubyguides.com/2018/11/ruby-2-6-new-features/
Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/57323
Also, play manual jobs once dependency jobs are done instead of polling
for the dependent jobs to be finished.
Signed-off-by: Rémy Coutable <remy@rymai.me>