Travis: Test on 2.4.0-preview2

.gitignore

@@ -8,3 +8,4 @@ spec/*.lock
 doc
 .yardoc
 Gemfile.lock
+gemfiles/Gemfile.2_4_0.lock

.travis.yml

@@ -17,6 +17,8 @@ matrix:
       gemfile: gemfiles/Gemfile.with_rack16
     - rvm: 2.3.1
       gemfile: gemfiles/Gemfile.with_rack2
+    - rvm: 2.4.0-preview2
+      gemfile: gemfiles/Gemfile.2_4_0
 
 notifications:
   email:

Gemfile

@@ -14,6 +14,10 @@ group :test do
   gem "coveralls", "~>0.8", require: false
   gem "simplecov", "~>0.10", require: false
   gem "codeclimate-test-reporter", "~>0.4"
-  gem "json"
+  if RUBY_VERSION > "2"
+    gem "json", "~> 2.0", ">= 2.0.2"
+  else
+    gem "json"
+  end
   gem "rspec", "< 4"
 end

gemfiles/Gemfile.2_4_0

@@ -0,0 +1,3 @@
+eval_gemfile File.expand_path('../../Gemfile', __FILE__)
+gem 'rack', '>= 2'
+