debian: add shebang to postrm, preinst files

The "Best Packaging Practices" says:
"the maintainer script must have a bash shebang line"
debian/gbp.conf: improve git-buildpackage config

- upstream-branch is "master" instead of gbp default "upstream"
- packages are made from an upstream tag, like v1.0.5
- debian-branch is in this case debian-mentors
- use xz compression (debian jessie default)