deleteProjectCard(cardId, cb) {
      return this._request('DELETE', `/projects/columns/cards/${cardId}`, null, cb);
   }
Delete a card
@see https://developer.github.com/v3/projects/cards/#delete-a-project-card
@param {string} cardId - the card to be deleted
@param {Requestable.callback} cb - will receive true if the operation is successful
@return {Promise} - the promise for the http request