title: Bootstrap 4 _bootstrap-variables.scss file is breaking compilation
body: <p>I just installed Bootstrap 4 using Bundler (gem 'bootstrap', '~> 4.0.0.alpha6') and created a new project with Compass by running <code>bundle exec compass create SProject1 -r bootstrap --using bootstrap</code> and here's what I got:</p>
  
  <pre class="lang-bash prettyprint-override"><code>directory SProject1/ 
  directory SProject1/javascripts/ 
  directory SProject1/javascripts/bootstrap/ 
  directory SProject1/sass/ 
  directory SProject1/stylesheets/ 
     create SProject1/config.rb 
     create SProject1/sass/styles.scss 
     create SProject1/sass/_bootstrap-variables.scss 
     create SProject1/javascripts/bootstrap/alert.js 
     create SProject1/javascripts/bootstrap/button.js 
     create SProject1/javascripts/bootstrap/carousel.js 
     create SProject1/javascripts/bootstrap/collapse.js 
     create SProject1/javascripts/bootstrap/dropdown.js 
     create SProject1/javascripts/bootstrap/modal.js 
     create SProject1/javascripts/bootstrap/popover.js 
     create SProject1/javascripts/bootstrap/scrollspy.js 
     create SProject1/javascripts/bootstrap/tab.js 
     create SProject1/javascripts/bootstrap/tooltip.js 
     create SProject1/javascripts/bootstrap/util.js 
     create SProject1/javascripts/bootstrap-sprockets.js 
     create SProject1/javascripts/bootstrap.js 
     create SProject1/javascripts/bootstrap.min.js 
      error SProject1/sass/styles.scss (Line 194 of SProject1/sass/_bootstrap-variables.scss: Undefined variable: "$grid-breakpoints".)
  Compilation failed in 1 files.
  </code></pre>
  
  <p>If I try to compile I get the same error (obviously):</p>
  
  <pre class="lang-bash prettyprint-override"><code>modified sass/styles.scss
      error sass/styles.scss (Line 194 of sass/_bootstrap-variables.scss: Undefined variable: "$grid-breakpoints".)
  </code></pre>
  
  <p>Here's my config.rb:</p>
  
  <pre class="lang-rb prettyprint-override"><code>require 'bootstrap'
  require 'compass/import-once/activate'
  # Require any additional compass plugins here.
  require 'autoprefixer-rails'
  
  # Set this to the root of your project when deployed:
  http_path = "/"
  css_dir = "stylesheets"
  sass_dir = "sass"
  images_dir = "images"
  javascripts_dir = "javascripts"
  
  # You can select your preferred output style here (can be overridden via the command line):
  # output_style = :expanded or :nested or :compact or :compressed
  
  # To enable relative paths to assets via compass helper functions. Uncomment:
  # relative_assets = true
  
  # To disable debugging comments that display the original location of your selectors. Uncomment:
  # line_comments = false
  
  
  # If you prefer the indented syntax, you might want to regenerate this
  # project again passing --syntax sass, or you can uncomment this:
  # preferred_syntax = :sass
  # and then run:
  # sass-convert -R --from scss --to sass sass scss &amp;&amp; rm -rf sass &amp;&amp; mv scss sass
  
  on_stylesheet_saved do |file|
    css = File.read(file)
    map = file + '.map'
  
    if File.exists? map
      result = AutoprefixerRails.process(css,
        from: file,
        to:   file,
        map:  { prev: File.read(map), inline: false })
      File.open(file, 'w') { |io| io &lt;&lt; result.css }
      File.open(map,  'w') { |io| io &lt;&lt; result.map }
    else
      File.open(file, 'w') { |io| io &lt;&lt; AutoprefixerRails.process(css) }
    end
  end
  </code></pre>
  
  <p>I only added the Autoprefixer thing as in the documentation, the rest of the files are as they came, haven't modified them yet as I can't even compile due to this weird error with the <strong>Undefined variable: "$grid-breakpoints"</strong>, I mean that's how the _bootstrap-variables.scss comes, so I am not sure what to do...</p>