update changelog for v6.0

Generated with [mdchangelog@0.2.0](https://github.com/creativelive/mdchangelog) command:
```
mdchangelog --no-prologue --no-orphan-issues --overwrite --remote spumko/hapi --order closed_at
```

I added some ordering options to mdchangelog and ordered the log by "closed_at". This will be a more predictable ordering than the previous order of "updated_at" which will change whenever folks comment or reference old issues. That's why this changelog has multiple diffs to the current one.