fix(uninstall): don't screw up global instance config on uninstall
fix(uninstall): don't screw up global instance config on uninstall

closes #220
- previously when you only had one instance and you uninstalled it, `ghost uninstall` would end up setting the instances value to an empty array rather than an empty object, resulting in a broken instance list for any future installs

TryGhost/Ghost-CLI,TomChantler/Ghost-CLI