TRIVIAL: Fix bug when deleting old version of change
TRIVIAL: Fix bug when deleting old version of change

Timestamp of PR is in seconds. So if two events triggering the
pipeline come in one second. Zuul will not delete the older one,
because the newer one is not an update of the older one according
to isUpdateOf(). Let's allow equality of timestamps in
isUpdateOf() to fix this.

gooddata/zuul,gooddata/zuul,gooddata/zuul,gooddata/zuul,gooddata/zuul