Upgraded my development machine from Ubuntu 14.10 to 15.04 and found an odd quirk with Transmission-gtk. The symptom was that Magnet links would not load from Firefox even though magnet is associated with transmission-gtk (to see this go to about:preferences#applications in Firefox and search for magnet)

I ran the transmission-gtk from a terminal and got,

transmission-gtk: error while loading shared libraries: libminiupnpc.so.8: cannot open shared object file: No such file or directory

This was strange as the packages depended upon libminiupnpc.so.10 so I then did a which transmission-gtk and it was /usr/local/bin/transmission-gtk and not the expected /usr/bin/transmission-gtk and I then remembered that I had manually installed Transmission version 2.83 on top of the older package manager version because the older package manager version of Transmission-gtk  in Ubuntu 14.10 would intermittently crash. As an aside if you really want the manually installed 2.83 version to run in Ubuntu 15.04 then you can symbolic link the relevant library with this,

cd /usr/lib/x86_64-linux-gnu
sudo ln -s libminiupnpc.so.10 libminiupnpc.so.8
sudo ldconfig -v

I eventually decided to uninstall the manually entered version by going to my source built directory and doing a sudo make uninstall

After you have uninstalled the manually entered version then a which transmission-gtk should return /usr/bin/transmission-gtk

The Transmission version with Ubuntu 15.04 is now 2.84 (onwards).

Once you have fixed it that the transmission-gtk can launch then clicking magnet links in Firefox now works.