deleteMilestone(milestone, cb) {
      return this._request('DELETE', `/repos/${this.__repository}/milestones/${milestone}`, null, cb);
   }
Delete a milestone (this is distinct from closing a milestone)
@see https://developer.github.com/v3/issues/milestones/#delete-a-milestone
@param {string} milestone - the id of the milestone to delete
@param {Requestable.callback} [cb] - will receive the status
@return {Promise} - the promise for the http request