Merge pull request #1687 from koic/release52

CI against Ruby 2.4.4

Author: koic

.travis.yml

@@ -35,7 +35,7 @@ language: ruby
 rvm:
   - 2.2.9
   - 2.3.6
-  - 2.4.3
+  - 2.4.4
   - 2.5.1
   - ruby-head
   - jruby-9.1.16.0