So it was the gmail notifier after all that I had installed according to this post (in Italian sorry)...
This is how I did it, thanks to everybody, I wouldn't have made it without you guys!
So according to what was returned by sudo apt-get update
and according to this post I unticked the similarly named PPA with the URL shown in the error message.
I re-ran what @EDi suggested sudo add-apt-repository ppa:ubuntugis/ubuntugis-unstable ; sudo apt-get update; sudo apt-get install libproj-dev
and it worked like a charm!
I spent so much time on this.... Thanks everybody!