Compute geowiki data only for full days

When running the cronjob on 2013-09-18 12:00, the job computed the
data for both 2013-09-17, and for 2013-09-18. The computed data for
2013-09-18 however can only contain edits up to 12:00 [1]. The data
cannot contain an edit made for example on 2013-09-18 14:00. Hence,
the last data point (i.e.: the data for the current date) might be
wrong. The data for 2013-09-18 gets recomputed (and corrected) when
running the cronjob on 2013-09-19. However, it's unnecessary to
compute the data for 2013-09-18 already on 2013-09-18, if we know that
it's bound to be stale anyways.

Hence, we shift the computed dates back one day. When running the job
on 2013-09-18 12:00, it now computes the data for 2013-09-16, and
2013-09-17. These days have been passed and their data is known to no
longer change.

[1] Or some minutes later depending on how long the job runs.

Change-Id: I7e194a60b2b5247d10601f14dfd5a583eac80239