lib/utils/getServerlessConfigFile.js/handleJsConfigFile
const handleJsConfigFile = jsConfigFile =>
  BbPromise.try(() => {
    // use require to load serverless.js
    // eslint-disable-next-line global-require
    const configExport = require(jsConfigFile);
    // In case of a promise result, first resolve it.
    return configExport;
  }).then(config => {
    if (_.isPlainObject(config)) {
      return config;
    }
    throw new Error('serverless.js must export plain object');
  });