Added minified `ghost.min.css` to gitignore
Added minified `ghost.min.css` to gitignore

- this is a minified version of `core/frontend/public/ghost.css`, but it
  is a release asset and shouldn't be committed to git
- without this change, the file will show up in git and people might
  accidentally commit it

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost