Keep using relish gem for documentation

See: #2071 (comment)

Author: benoittgt

Gemfile

@@ -13,6 +13,7 @@ gem 'yard', '~> 0.8.7', :require => false
 group :documentation do
   gem 'redcarpet',     '2.3.0'
   gem 'github-markup', '1.0.0'
+  gem 'relish'
 end
 
 platforms :jruby do