Compiles SCSS into CSS

@returns {Promise}
function compileScss() {
  return new Promise((resolve, reject) => {
      sass.render({
        file: "sass/qooxdoo.scss",
        outFile: "build/qooxdoo.css"
      }, function(err, result) {
        if (err)
          reject(err);
        else
          resolve(result);
      });
    })
    .then(result => fs.writeFileAsync("build/qooxdoo.css", result.css, "utf8"));
}