Update README.md, link for rubyonrails.org

remove extra 'http://'
Merge pull request #56 from renchaoevee/patch-1

Update README.md, link for rubyonrails.org
fix generated_image_url when generated_images_dir is set
Merge branch 'fix_generated_image_url' of github.com:s12chung/compass-rails into rails4
updated compass dependency to latest alpha for testing