Fix sprocket errors by applying sass

This pull request fixes an error we receive when precompling assets in production mode:

    NoMethodError: undefined method `[]' for nil:NilClass
      (in /home/frank/Source/ruby/accompli-tool/app/assets/stylesheets/application.scss)
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sprockets-2.11.3/lib/sprockets/sass_functions.rb:63:in `sprockets_context'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-rails-4.0.4/lib/sass/rails/helpers.rb:23:in `asset_url'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/script/funcall.rb:113:in `_perform'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/script/node.rb:40:in `perform'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/script/list.rb:71:in `block in _perform'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/script/list.rb:71:in `map'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/script/list.rb:71:in `_perform'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/script/node.rb:40:in `perform'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/tree/visitors/perform.rb:298:in `visit_prop'
    /home/frank/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/sass-3.2.19/lib/sass/tree/visitors/base.rb:37:in `visit'