Time unit off-by-one fix for #65

```
2014-04-15T11:20:10.209 INFO                  index_loop:291  Would have attempted deleting index logstash-2014.04.13 because it is 2 days, 0:00:00 older than the calculated cutoff.
2014-04-15T11:20:10.209 INFO                  index_loop:291  Would have attempted deleting index logstash-2014.04.14 because it is 1 day, 0:00:00 older than the calculated cutoff.
2014-04-15T11:20:10.209 INFO        find_expired_indices:209  logstash-2014.04.15 is 0:00:00 above the cutoff.
```
Merge pull request #66 from untergeek/time-off-by-one-patch

Time unit off-by-one fix for #65