nodelet_core: 1.8.4-0 in 'hydro/distribution.yaml' [bloom]

Increasing version of package(s) in repository `nodelet_core` to `1.8.4-0`:

- distro file: `hydro/distribution.yaml`
- bloom version: `0.5.9`
- previous version for package: `1.8.2-0`

## nodelet

```
* Unload nodelets if they fail to initialize
  Fixes #13 <https://github.com/ros/nodelet_core/issues/13>
```

## nodelet_core

- No changes

## nodelet_topic_tools

- No changes