Bump ruby versions in Travis CI

[giraffate]

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/

[giraffate]

CI using gemfiles/sidekiq_develop.gemfile failed in this PR. However, looks like it's fixed by sidekiq/sidekiq#4301.

[mhenrixon]

If you have a look in master branch I allow failures on sidekiq_develop.gemfile