Update rubies in .travis.yml

Author: ttanimichi

.travis.yml

@@ -7,8 +7,9 @@ cache:
   bundler: true
   yarn: true
 rvm:
-  - 2.5.0
-  - 2.3.0
+  - 2.5.1
+  - 2.3
+  - ruby-head
   - jruby-9.1.9.0
 
 gemfile:
@@ -44,6 +45,7 @@ matrix:
       gemfile: gemfiles/rails_5_no_sprockets.gemfile
 
   allow_failures:
+    - rvm: ruby-head
     - rvm: jruby-9.1.9.0
 
 before_install: