[Sass] Load Sass::Plugin::Rack into Rails.configuration, not Metal in Rails 3.
[Sass] Load Sass::Plugin::Rack into Rails.configuration, not Metal in Rails 3.

Closes gh-190

macressler/haml,srinivashappy/sass,Nzaga/sass,StephanieMak/sass,keithpitty/haml,nickg33/sass,Galactix/sass,Ahnita/sass,chriseppstein/sass,stewx/sass,baiyanghese/sass,tangposmarvin/haml,xzyfer/sass,tenderlove/sass,stanhu/haml,rammy7219/sass,askl56/sass,Industrys/sass,askl56/sass,OrenBochman/sass,cmpereirasi/sass,haml/haml,Industrys/sass,schneems/sass,tangposmarvin/sass,cmpereirasi/sass,hcatlin/sass,chriseppstein/haml,dylannnn/sass,Iamronan/sass,sideci-sample/sideci-sample-haml,dg1dd1ngs/haml,srawlins/sass,schneems/sass,Nzaga/sass,concord-consortium/haml,modulexcite/sass,haml/haml,Galactix/sass,hcatlin/sass,HugoRLopes/sass,srawlins/sass,nickg33/sass,MetaAshley/sass,uniba/hamldoc_ja,antonifs/sass,Anshdesire/sass,14113/sass,haml/haml,JuanitoFatas/sass,briangonzalez/sass,Iamronan/sass,lmetla/sass,rammy7219/sass,StephanieMak/sass,PMArtz92/sass,tangposmarvin/haml,nazar-pc/haml,ridixcr/sass,ridixcr/sass,nicklima/sass,JuanitoFatas/sass,oss-practice/sass,macressler/haml,Anshdesire/sass,danielfein/sass,antonifs/sass,amatsuda/haml,OrenBochman/sass,lmetla/sass,liquidmetal/sass,jaroot32/sass,zaru/sass,DanteKaris/sass,tangposmarvin/sass,PMArtz92/sass,dylannnn/sass,saper/sass,keithpitty/haml,liquidmetal/sass,saper/sass,nicklima/sass,amitsuroliya/sass,stanhu/haml,nazar-pc/haml,MetaAshley/sass,xzyfer/sass,dg1dd1ngs/haml,HugoRLopes/sass,zaru/sass,Ahnita/sass,Andrey-Pavlov/sass,haml/haml,amitsuroliya/sass,jaroot32/sass,danielfein/sass,Andrey-Pavlov/sass,chriseppstein/haml,stewx/sass,srinivashappy/sass,DanteKaris/sass,modulexcite/sass