Merge pull request #431 from masarufuruya/master

[CI] Test against Ruby 2.3.7, 2.4.4, 2.5.1

Author: pixeltrix

.travis.yml

@@ -11,10 +11,10 @@ rvm:
   - 1.9
   - 2.0
   - 2.1
-  - 2.2
-  - 2.3.6
-  - 2.4.3
-  - 2.5.0
+  - 2.2.10
+  - 2.3.7
+  - 2.4.4
+  - 2.5.1
   - ruby-head
   - jruby-19mode
   - rbx