roscpp_core: 0.4.4-0 in 'hydro/distribution.yaml' [bloom]

Increasing version of package(s) in repository `roscpp_core` to `0.4.4-0`:

- upstream repository: git@github.com:ros/roscpp_core.git
- release repository: https://github.com/ros-gbp/roscpp_core-release.git
- distro file: `hydro/distribution.yaml`
- bloom version: `0.5.16`
- previous version for package: `0.4.3-0`

## cpp_common

- No changes

## roscpp_serialization

- No changes

## roscpp_traits

- No changes

## rostime

```
* move implementation of Duration(Rate) constructor (#30 <https://github.com/ros/roscpp_core/issues/30>)
* fix Duration initialization from seconds for negative values  (#29 <https://github.com/ros/roscpp_core/pull/29>)
```