Bump ruby versions in Travis CI (#425)
Bump ruby versions in Travis CI (#425)

Reference:
- https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-6-4-released/
- https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-5-6-released/
- https://www.ruby-lang.org/en/news/2019/08/28/ruby-2-4-7-released/
mhenrixon/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs,mhenrixon/sidekiq-unique-jobs