.travis.yml in yao-0.3.1 vs .travis.yml in yao-0.3.2

@@ -1,10 +1,11 @@
 language: ruby
 
 rvm:
-  - 2.1.7
-  - 2.2.3
-  - 2.3.0-preview1
+  - 2.2.7
+  - 2.3.4
+  - 2.4.1
+  - ruby-head
 
 install: bundle install --path vendor/bundle
 
 cache:
   directories: vendor/bundle