Requires YAML to read the configuration

In Ruby 2.1.2 with Rails 4.1.4 yaml wasn't being required and I was seeing the following error:

    gems/sunspot_rails-2.1.1/lib/sunspot/rails/configuration.rb:360:in `block in user_configuration': uninitialized const$
nt Sunspot::Rails::Configuration::YAML (NameError)

Just had to require YAML, and everything works well :)