Update travis.yml

- Add ruby 2.5
- Update versions of 2.4, 2.3, 2.2, 2.1

Author: mrkn

.travis.yml

@@ -3,9 +3,11 @@ language: ruby
 
 rvm:
   - ruby-head
-  - 2.3.0
-  - 2.2.4
-  - 2.1
+  - 2.5.0
+  - 2.4.3
+  - 2.3.5
+  - 2.2.9
+  - 2.1.10
 
 before_install:
   - gem update --system