Comment 47 for bug 1594959

It might be useful to add how to properly remove the PPA. Don't just disable or remove it from your sources or the packages will stay. Install ppa-puge if it's not already:

sudo apt-get install ppa-purge

then, use it to disable the PPA and rollback the packages to the version from the Ubuntu repos:

sudo ppa-purge ppa:monsta/testing

Finally, you can remove it (it will be disabled) from the Software Sources.