Update to check median instead of last value.

The check by graphite-data.rb was only checking the latest value
returned by the graphite target, which in certain corner cases was
giving false positives for:

```
asPercent(metrics.:::metric_prefix:::.memory.memory.used,sum(metrics.:::metric_prefix:::.memory.memory.*))
asPercent(metrics.:::metric_prefix:::.swap.swap.used,sum(metrics.:::metric_prefix:::.swap.swap.*))
```

... with the percentage returned being '100.0', despite this being very
*extremely* unlikely. Unfortunately, it's a very intermittent problem so
the exact cause of this is unknown.

Returning the median value should avoid this outlier.

This should also make this check more useful for rapidly changing
values, such as CPU checks.