Ghost is wonderful, but every time they publish a new version there are too many steps to perform in order to complete the upgrade.
Since doing it manually is boring and prone to errors, I created an easy script that does the dirty job for you.
It only needs the version number you want to upgrade to and you're done:

./update_ghost.sh 0.11.4

Of course it doesn't check version conflicts or solve problems on your behalf (back up your posts!), but for most updates (tipically minor versions) it will work great.
I uploaded the source code into a gist so it's versionable, forkable and open to improvements :)
Here you can find the repo; if you are lazy, just grab the source code below.

Update (2016-10-02): I added a manual run of npm and the trap of SIGINT signal. This way, DB migration through different versions can be correctly performed and the script can terminate normally, even after the CTRL+C.

Update (2017-01-22): Fixed path and name of downloaded file.