Remove 'images' folder from precompile list

Not including the 'assets/images' folder fixes precomilation in Rails 4.2.0.beta1. Since the folder doesn't exist a `Errno::ENOENT (No such file or directory @ realpath_rec - /Users/alex/.rvm/gems/ruby-2.1.1/gems/bootstrap-sass-3.2.0.1/assets/images` is raised during precompilation.