Estimate

I noticed while writing [with_progress](http://github.com/marcandre/with_progress) (which you might want to consider including in this gem?) that the time estimations were off.
The progress estimation is good, it's only the remaining time calculation that is incorrect. This PR fixes it.

The spec mimics a loop with iterations taking exactly 0.5 seconds.
The improvement should be quite obvious, as without the fix, the test fails with:

```
  expected: [" ETA: 00:00:05", " ETA: 00:00:04", " ETA: 00:00:04", " ETA: 00:00:03", " ETA: 00:00:03", " ETA: 00:00:02", " ETA: 00:00:02", " ETA: 00:00:01", " ETA: 00:00:01", " ETA: 00:00:00"]
       got: [" ETA: 00:00:00", " ETA: 00:00:04", " ETA: 00:00:02", " ETA: 00:00:03", " ETA: 00:00:02", " ETA: 00:00:02", " ETA: 00:00:01", " ETA: 00:00:01", " ETA: 00:00:00", " ETA: 00:00:00"] (using ==)
```

Thanks for `ruby-progressbar`