Fix global stats: compute for previous day (bug 1088639)

Fixes [bug 1088639](https://bugzilla.mozilla.org/show_bug.cgi?id=1088639)

To backfill, you can run the following script:

```
import datetime

import manage
import cronjobs

from django.core.management import call_command


start = datetime.date(year=2014, month=7, day=10)
today = datetime.date.today()

current = start

while current < today:
    call_command('cron', 'update_global_totals', current.strftime('%Y-%m-%d'))
    # Manually override the django-cronjobs lock. Yikes.
    cronjobs.registered_lock.clear()
    current += datetime.timedelta(days=1)
```