Use a more recent rubygems from squeeze-backports

The version of rubygems in squeeze is old enough that it won't
work with our Gemfile / Gemfile.lock.  Fortunately, the one in
squeeze-backports seems to work OK.
Invoke commands as the unprivileged user in a login shell

On some setups, the unprivileged user's .bashrc file won't be
sourced unless you've started a login shell - we need that
to be sourced to add the gem's bin directory to the PATH,
or "bundle" won't be found, for example.