update interpreter list in .travis.yml

Author: Ben Miller

.travis.yml

@@ -3,8 +3,11 @@ script: bundle exec rake test
 rvm:
   - 1.9.3
   - 2.0.0
-  - 2.1.0
-  - 2.2.0
+  - 2.1.10
+  - 2.2.8
+  - 2.3.6
+  - 2.4.3
+  - 2.5.0
   - ruby-head
   - jruby-head