Switched to Git-GUI git client

Since GitKraken is no longer free for commercial use](https://www.gitkraken.com/pricing) it’s been replaced in the GantSign development environment.

Installed in place of GitKraken are Git-GUI and Gitk; while they’re a lot less pretty, they are more stable, feature complete and are open source.

For more details of Git-GUI and Gitk support view the features page.