GitRef is No Longer Optional for CaC-Based Projects; Update DeploymentProcessService.Get to Reflect This Change

The `gitRef` property is now required when created CacC-based projects. This change must be reflected in the underlying service (`DeploymentProcessService`) -- namely, the `Get` function. This function must be updated.

🔗 https://github.com/OctopusDeploy/OctopusCLI/pull/168