Ubuntu PPA release candidate builds would overwrite previous stable releases

This hasn't been a real problem yet, but the first builds from release_0.2.8.3 landed in the new LTS ppa as armagetronad0.2.8.3.6rcz1398ppa1, which whould have been considered better than armagetronad0.2.8.3.5, which luckily is in the main PPA. But any build from branch release_0.2.9 would immediately outdate armagetronad0.2.8.3.5 there.

They should either be rebranded armagetronad-rc or not uploaded to the PPA at all.