fix(backup): do not backup casper symlink
fix(backup): do not backup casper symlink

refs: https://github.com/TryGhost/Toolbox/issues/334
refs: https://github.com/TryGhost/Ghost-CLI/issues/468

- in Ghost CLI casper is always a symlink
- we don't need to back it up, it will always be present and updated

TryGhost/Ghost-CLI