trivial: travis: re-enable ubuntu CI builds
trivial: travis: re-enable ubuntu CI builds

The efivar transition finished there.

fwupd/fwupd,vathpela/fwupd,superm1/fwupd,vathpela/fwupd,fwupd/fwupd,hughsie/fwupd,vathpela/fwupd,superm1/fwupd,fwupd/fwupd,hughsie/fwupd,vathpela/fwupd,superm1/fwupd,hughsie/fwupd,hughsie/fwupd,fwupd/fwupd