bond_core: 1.7.15-0 in 'hydro/distribution.yaml' [bloom]

Increasing version of package(s) in repository `bond_core` to `1.7.15-0`:

- upstream repository: git://github.com/ros/bond_core.git
- release repository: https://github.com/ros-gbp/bond_core-release.git
- distro file: `hydro/distribution.yaml`
- bloom version: `0.5.12`
- previous version for package: `1.7.14-0`