Build flatpak package with fedpkg

[Chenxiong Qi]

Hi,

There is a new rpkg release, which has a new command flatpak-build for
building flatpak package.

Updates:
https://bodhi.fedoraproject.org/updates/rpkg-1.56-2.fc29
https://bodhi.fedoraproject.org/updates/rpkg-1.56-2.fc28

Happy packaging!

--
Regards,
Chenxiong Qi