Mendeley 1.17.9 is now out, and 1.17.8 (the version in the Portage tree) is no longer available for download. https://www.mendeley.com/release-notes/
1.17.9 is in the portage and this Ticket can be closed. For future reference, it is possible to download old versions of mendeleydesktop by changing the url: 1.17.8: https://desktop-download.mendeley.com/download/linux/mendeleydesktop-1.17.8-linux-x86_64.tar.bz2 1.17.9: https://desktop-download.mendeley.com/download/linux/mendeleydesktop-1.17.9-linux-x86_64.tar.bz2 1.17.10: https://desktop-download.mendeley.com/download/linux/mendeleydesktop-1.17.10-linux-x86_64.tar.bz2
Cool. It would be nice if these instructions to download older versions were included in the ebuild, so it didn't break every time they release a new version, but I guess this isn't the place to make that happen.