Bug in latency calculation or tracking

Just ran 15k requests against a single app, using a single request:

```
2014/01/21 06:08:37 Vegeta is attacking 1 targets in random order for 30s...
2014/01/21 06:09:07 Done!
2014/01/21 06:09:07 Writing results to 'stdout'...
Requests        [total]                 15000
Duration        [total]                 30.010003254s
Latencies       [mean, 95, 99, max]     5.28031ms, 2.329778ms, 3.002067645s, 3.002067645s
Bytes In        [total, mean]           9950012, 663.33
Bytes Out       [total, mean]           0, 0.00
Success         [ratio]                 100.00%
Status Codes    [code:count]            200:15000  
Error Set:
```

How can the 95th percentile be lower than the average?