added missing yaml require
added missing yaml require

--HG--
extra : convert_revision : svn%3A177060d3-6165-4ab0-95eb-642399a075a8/nanoc/trunk%40596

legallybrown/nanoc,hilohiro/nanoc,remko/nanoc,nanoc/nanoc,RubenVerborgh/nanoc,gjtorikian/nanoc,barraq/nanoc