Test against Ruby 2.1 - 2.4
diff --git a/.travis.yml b/.travis.yml
index 8a9593e..27e1dd1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,8 @@
 language: ruby
 rvm:
-  - 1.9.3
   - 2.1.5
-  - 2.2.1
+  - 2.2.7
+  - 2.3.4
+  - 2.4.1
 script:
   - rake