Make backward-compatible .deb packages, accepted by BinTray
After recent sbt / sbt-native-packager updates, it turns out that BinTray does not accept the .deb packages build by our build process. Closer examination reveals that the problem is subtle change of .deb format, which evolved from traditional .tar.gz packing => .tar packing => .tar.xz packing. I've contacted BinTray support, and they've confirmed that they currently only support .tar.gz, and plans to support more formats are there, but these are pretty long-term goals.

For gory details, see `dpkg-buildpackage` and `dpkg-source` manpages (`-z` and `-Z` options). Large changes came around dpkg 1.15.5.

I've picked up dialogue at sbt-native-packager — https://github.com/sbt/sbt-native-packager/issues/1067 — hopefully I'll find enough stamina to fix eventually.

This is a blocker for v0.8, as we are currently unable to build .deb packages.