Add test of combination of Ruby 2.1 and Rails 5.0 / 5.1

Author: y-yagi

.travis.yml

@@ -24,6 +24,10 @@ matrix:
     - rvm: ruby-head
   fast_finish: true
   include:
+    - gemfile: gemfiles/Gemfile-rails-5-0
+      rvm: 2.2.7
+    - gemfile: gemfiles/Gemfile-rails-5-1
+      rvm: 2.2.7
     - gemfile: gemfiles/Gemfile-rails-5-0
       rvm: 2.3.4
     - gemfile: gemfiles/Gemfile-rails-5-1