Merge pull request #443 from aycabta/travis-ruby2.4.1

Use Ruby 2.4.1 for Travis CI

Author: hsbt

.travis.yml

@@ -11,7 +11,7 @@ rvm:
   - 2.1.10
   - 2.2.6
   - 2.3.3
-  - 2.4.0
+  - 2.4.1
   - ruby-head
   - rbx-2
   - jruby-1.7.25