Comment 44 for bug 1371783

If you want to work around this because the prompt is annoying, here's how:

Download the .exe files from here and put them in your Downloads folder:

https://sourceforge.net/projects/corefonts/files/the%20fonts/final/

Afterwards, run the following two commands:

sudo cp ~/Downloads/*.exe /var/lib/update-notifier/package-data-downloads/partial/
gksudo /usr/lib/update-notifier/package-data-downloader

That should work and the prompt will go away.