Increase valid number of digits for intervals

Postgres will store and return more hours than just two digits:

``` 
will=# select * from deltas order by id desc limit 2;
 id  |   delta
-----+-----------
 336 | 167:30:00
 335 | 167:00:00
(2 rows)

will=# select '1000000000 hours'::interval;
     interval
------------------
 1000000000:00:00
(1 row)
```