update changelog

updated changelog using [mdchangelog](https://www.npmjs.org/package/mdchangelog) command
```
mdchangelog --no-prologue --no-orphan-issues --overwrite --remote hapijs/hapi --order-milestones semver --order-issues closed_at
```

If you're happy with the format of the changelog, maybe the command could be run as part of a build process (at least for major releases - taking note of github api rate-limiting) otherwise I will open PRs with a new changelog every-so-often