Merge pull request ruby#244 from aycabta/2.5.0

Use 2.5.0 and more latest Ruby versions

Author: hsbt

.travis.yml

@@ -3,9 +3,10 @@ sudo: false
 rvm:
   - 2.0.0
   - 2.1.10
-  - 2.2.8
-  - 2.3.5
-  - 2.4.2
+  - 2.2.9
+  - 2.3.6
+  - 2.4.3
+  - 2.5.0
   - ruby-head
   - jruby-9.1.15.0
   - jruby-head