bundle rack 2 that is required by rails 5

stay on rack 1 only on ruby 1.x that is not supported by rack 2

Author: amatsuda

Gemfile

@@ -1,12 +1,13 @@
 source 'https://rubygems.org'
 
 gem 'rails', group: :test
-gem 'rack', '~>1.6', group: :test
 gem "mime-types", "< 3", group: :test
 
 if RUBY_VERSION > '2'
+  gem 'rack'
   gem 'json', '>= 2'
 else
+  gem 'rack', '~>1.6'
   gem 'json', '~> 1.8.0'
 end