Custom progress time displays 2 mins after 60 seconds

The elapsed time for a custom format will jump straight to `2 mins` after it goes over 60 seconds. 
### Test

``` python
progress = self.progress_bar(60)
progress.set_format(' [%bar%] Elapsed: %elapsed%')
progress.start()
count = 0
while True:
    if count == 70:
        progress.finish()
        break
    count += 1
    progress.advance()
    time.sleep(1)
```
