Add GEM_HOME and GEM_ROOT to GEM_PATH

Rubygems will automatically look in GEM_HOME and GEM_PATH, so I didn't
think I needed to put GEM_HOME in GEM_PATH, but I think that might have
been screwing some stuff up.

Rubinius installs all the rubysl-* gems in GEM_ROOT (and I wasn't using
GEM_ROOT for anything else), so I think that should be on the path too.