mavros: 0.8.4-0 in 'hydro/distribution.yaml' [bloom]

Increasing version of package(s) in repository `mavros` to `0.8.4-0`:

- upstream repository: https://github.com/mavlink/mavros.git
- release repository: https://github.com/mavlink/mavros-release.git
- distro file: `hydro/distribution.yaml`
- bloom version: `0.5.12`
- previous version for package: `0.8.3-0`

## libmavconn

```
* Fix libmavconn deps.
  Releases 0.9 and 0.8.3 ar broken because i forgot to add mavlink dep.
* Contributors: Vladimir Ermakov
```

## mavros

- No changes

## mavros_extras

- No changes