BigQuery logs are labeled in UTC time, so query them in UTC time as well.

Summary:
Currently there is an 8-hour time gap at the end of each day when we're still
querying today's logs from BigQuery even though we have all of today's and
BigQuery is already storing tomorrow's (because it's using UTC time). This
changes the cron job to request logs for the current day UTC so we get the new
logs immediately.

Test Plan:
Tested `datetime.datetime.utcnow().strftime("%Y%m%d")` in ipython to make
sure it does what I expect.

Reviewers: csilvers

Reviewed By: csilvers

Subscribers: jace

Differential Revision: https://phabricator.khanacademy.org/D15129