Buffer all files before parsing them to livingcss.
function bufferContents(file, enc, cb) {
    // ignore empty files
    if (file.isNull()) {
      return cb();
    }

    // no streams
    if (file.isStream()) {
      this.emit('error', new PluginError('gulp-livingcss', 'Streaming not supported'));
      return cb();
    }

    files.push(file.path);

    cb();
  }