Add gemfile for Rails 6.0

* The latest version of Rails 6.0 is Rails 6.0.0.rc2
* Rails 6.0 requires Ruby 2.5.0 then excluded CI agains Ruby 2.4 or
lower

Refer Rails 6.0 required_ruby_version
https://github.com/rails/rails/blob/16f2cd88e3ee92de341ba30b102e9206be3c68a7/rails.gemspec#L12

Author: yahonda

.travis.yml

@@ -16,9 +16,14 @@ gemfile:
   - gemfiles/4.2.gemfile
   - gemfiles/5.0.gemfile
   - gemfiles/5.2.3.gemfile
+  - gemfiles/6.0.gemfile
 matrix:
   fast_finish: true
   exclude:
+    - rvm: 2.2.6
+      gemfile: gemfiles/6.0.gemfile
+    - rvm: 2.3.3
+      gemfile: gemfiles/6.0.gemfile
     - rvm: 2.4.1
       gemfile: gemfiles/3.2.gemfile
     - rvm: 2.4.1
@@ -27,6 +32,8 @@ matrix:
       gemfile: gemfiles/4.1.gemfile
     - rvm: 2.4.1
       gemfile: gemfiles/4.2.gemfile
+    - rvm: 2.4.1
+      gemfile: gemfiles/6.0.gemfile
     - rvm: 2.5.1
       gemfile: gemfiles/3.2.gemfile
     - rvm: 2.5.1

gemfiles/6.0.gemfile

@@ -0,0 +1,15 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "rake"
+gem "appraisal"
+gem "activesupport", "~> 6.0.0.rc2"
+gem "addressable", "~> 2.2"
+gem "codeclimate-test-reporter", :group => :test, :require => nil
+
+group :development, :test do
+  gem "byebug", :platforms => [:mri_25]
+end
+
+gemspec :path => "../"