fixed wrong ruby name in travis

hsbt · hsbt · commit c480800de0de · 2016-03-11T16:46:23.000+09:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,4 +1,4 @@
 language: ruby
 rvm:
-  - head
+  - ruby-head
 before_install: gem install bundler -v 1.11.2
