Fix cap rake:themes:install when ran by itself.

If we try to do:

    cap rake:themes:install

it fails, because we're using release_path in the code.

In capistrano, release_path is always the "release currently being created".
When running cap rake:themes:install alone, we're not creating a new release,
but simply updating the themes. So, what really matters for us is the
current_path.