It doesn't look like the Ruby library currently exposes this. But the source code is here: https://github.com/dropbox/dropbox-sdk-ruby/blob/master/lib/dropbox_sdk.rb#L1222-L1225. If you're up for it, please submit a pull request to add this to the SDK.
In the meantime, here's standalone code to do this:
def long_share_url(access_token, path)
client = DropboxClient.new(access_token)
session = DropboxOAuth2Session.new(access_token, nil)
response = session.do_get "/shares/auto/#{client.format_path(path)}", {"short_url"=>false}
Dropbox::parse_response(response)
end