Use latest Rubies on Travis CI
Use latest Rubies on Travis CI

These Rubies has been released.

- https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-5-1-released/
- https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-4-4-released/
- https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-3-7-released/
- https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-2-10-released/

bkeepers/dotenv