Update MRI Rubies on Travis CI, adds Ruby 2.5 and Ruby 2.6

[aried3r]

No description provided.

[olleolleolle]

• 2.5.5 https://www.ruby-lang.org/en/news/2019/03/15/ruby-2-5-5-released/
• 2.6.2 https://www.ruby-lang.org/en/news/2019/03/13/ruby-2-6-2-released/

[olleolleolle]

• JRuby 9.2.7.0 release blog post

[olleolleolle]

2.4.9, 2.5.7, 2.6.5 released

[aried3r]

I believe this repo is unmaintained and further PRs should go to the caxlsx repo.

And maybe for testing it will be ok to not use the latest versions, but save time on CI using the rubies that come with bionic. https://docs.travis-ci.com/user/reference/bionic/#ruby-support

WDYT, @olleolleolle, since you also open a lot of PRs for gems. :)