update ghost download url

[georgkrause]

There are some issues with this and the following line: https://github.com/dictvm/shellscripts/blob/master/update-ghost.sh#L36

1. The Download URL ist now https://github.com/TryGhost/Ghost/releases/download/$VERSION/Ghost-$VERSION.zip
2. The downloaded file is named Ghost-$VERSION.zip, not ghost-$VERSION.zip

With these changes the Script is working again. Thanks for your work!