Merge pull request #536 from aycabta/use-ruby-2.4.2-2.3.5-2.2.8

Use Ruby 2.4.2, 2.3.5, 2.2.8

Author: yuki24

.travis.yml

@@ -3,9 +3,9 @@ before_install:
   - gem install bundler --no-document
 language: ruby
 rvm:
-  - 2.2.7
-  - 2.3.4
-  - 2.4.1
+  - 2.2.8
+  - 2.3.5
+  - 2.4.2
   - ruby-head
   - jruby-9.1.13.0
 env: