hector_gazebo: 0.3.1-0 in 'hydro/release.yaml' [bloom]

Increasing version of package(s) in repository `hector_gazebo`:
- previous version: `0.3.0-2`
- new version: `0.3.1-0`
- distro file: `hydro/release.yaml`
- bloom version: `0.4.4`