build rpms/debs for all components at once

FPM can build rpm and deb packages from our gem dependencies.
FPM should be installed with:

  gem install fpm

The Gemfile contains a list of all runtime dependencies for all Libertree
components (except the frontend).  We collect all dependent gems with bundler:

  bundle install --without development --path vendor
  bundle pack --all

The dependent gem packages are configured to install to /usr/share/libertree/gems;
the backend libraries, config files and scripts will be installed to
/usr/share/libertree/backend-rb .

TODO: build packages for the frontend