Use America/Los_Angeles timezone for queries

Right now, the 'current' day rolls over when UTC-time passes into the
next calendar day.  Given the maximum offset from UTC in
America/Los_Angeles is eight hours, the current day is hidden pretty
early (4 or 5 PM, depending on PST/PDT switch), which can get in the
way of checking in guests for evening activities.

So, use the 'America/Los_Angeles' time zone, which will automatically
adjust given the time of year to apply the right offsets when
truncating to the "date" type.  Rollovers will happen at midnight,
relative to Heroku HQ.