Change active_url to url

I just "degraded" that validation to be less restricted.
The reason is that it works on
Linux, but doesn't work under Windows if you don't have the PEAR package
installed.

http://laravel.com/docs/validation#rule-url
http://php.net/manual/en/function.checkdnsrr.php#refsect1-function.checkdnsrr-notes

Signed-off-by: Simone <hello@simonewebdesign.it>