Change general Linux language to Ubuntu in list of desktop apps

This MR should update some language on https://gitter.im/apps from linux to ubuntu improving user experience. I myself was disappointed at least twice because I thought there was already a desktop app for Fedora Linux. I wasn't sure whether your .deb packages also supports Fedora. In this case, you might rename everything back to debian.

I'll be looking into testing https://gitlab.com/gitlab-org/gitter/desktop/merge_requests/196 to obtain .rpm files (as I'm using fedora ;) )