Nonce is too small

Hi, When I tried this code
```
$bfx = new Bitfinex($api_key, $api_secret);

//var_dump($bfx->get_symbols());

$account_summary = $bfx->get_summary();

var_dump($account_summary);
```
It results to this
array(2) { ["message"]=> string(19) "Nonce is too small." ["error"]=> bool(true) }