travis: update Rubies

Author: kou

.travis.yml

@@ -4,8 +4,8 @@ notifications:
 sudo: false
 language: ruby
 rvm:
- - 2.3.6
- - 2.4.3
- - 2.5.0
+ - 2.4
+ - 2.5
+ - 2.6
  - ruby-head
 before_install: gem install bundler