Solve Syck:Emitter NoMethodError 

In Rails 3.1, metric-fu fails to run (via metrical) due to the following error:

`/Users/rambo/.rvm/rubies/ruby-1.9.2-p180/lib/ruby/1.9.1/psych/visitors/emitter.rb:17:in `end_document': undefined method `write' for #<Syck::Emitter:0x007fcc4120b498> (NoMethodError)`

Including `require psych` solves this problem.