You can see the reason here: https://github.com/resque/resque/issues/1143 -- it boils down to a temporary bug that is fixed, but not released yet. I fixed mine by adding
require 'yaml'
at the top of the server.rb file
note that the server.rb file on my machine was located at: /usr/local/lib/ruby/gems/2.0.0/gems/resque-1.25.1/lib/resque/server.rb
your location may vary