Make a request to retrieve Wharton GSR listings.

def get_wharton_gsrs(self, sessionid, date=None):
        """ Make a request to retrieve Wharton GSR listings. """
        if date:
            date += " {}".format(self.get_dst_gmt_timezone())
        else:
            date = datetime.datetime.utcnow().strftime("%Y-%m-%d %H:%S")
        resp = requests.get('https://apps.wharton.upenn.edu/gsr/api/app/grid_view/', params={
            'search_time': date
        }, cookies={
            'sessionid': sessionid
        })
        if resp.status_code == 200:
            return resp.json()
        else:
            raise APIError('Remote server returned status code {}.'.format(resp.status_code))