code: function getLabels(prs) {
    return Promise.all(prs.reduce((flattenedPRs, pr) => {
      if (pr) {
        return flattenedPRs.concat(pr);
      }
    }, []).map(populateLabelsForPR));
  }
docs: Fetch labels for a list of pull-requests
  @param  {Array}  repos List of pull-requests
  @return {Promise}      Resolves to an array of PRs containing labels