Apply 'classic' sass config setting for generating line comments

This PR just adds the line_comments option to the sass options so we can get the mapping from the generated css to the original sass files:

    /* line 246, /foo/bar/baz/gems/zurb-foundation-4.2.3/scss/foundation/components/_global.scss */
    html,
    body {
      font-size: 16px; }