gazebo_ros_pkgs: 2.3.1-0 in 'hydro/release.yaml' [bloom]

Increasing version of package(s) in repository `gazebo_ros_pkgs`:
- previous version: `2.3.0-0`
- new version: `2.3.1-0`
- distro file: `hydro/release.yaml`
- bloom version: `0.4.4`