Suppress warning on is_readable for servers with open_basedir restrictions.

Servers with open_basedir throw an error on this line: 

`if (is_readable('/dev/urandom') &&`

This patch suppresses the error. The same fix has also made in the original PHPass. See [here](http://cvsweb.openwall.com/cgi/cvsweb.cgi/projects/phpass/PasswordHash.php?annotate=1.8) and [here](http://cvsweb.openwall.com/cgi/cvsweb.cgi/projects/phpass/PasswordHash.php?r1=1.8#rev1.8).