gencpp: 0.5.1-0 in 'indigo/distribution.yaml' [bloom]

Increasing version of package(s) in repository `gencpp` to `0.5.1-0`:

- distro file: `indigo/distribution.yaml`
- bloom version: `0.5.1`
- previous version for package: `0.5.0-0`

## gencpp

```
* use catkin_install_python() to install Python scripts (#18 <https://github.com/ros/gencpp/issues/18>)
* add 'u' suffix to unsigned enum values to avoid compiler warning (#16 <https://github.com/ros/gencpp/issues/16>)
```