update .info file to reflect that schema requires PHP > 5.1

After enabling this with PHP 4.4.7, I get the fatal error:

Fatal error: Call to undefined function: array_intersect_key() in modules/schema/schema.module on line 274

checking the PHP docs:
http://www.php.net/manual/en/function.array-intersect-key.php

which states: PHP 5 >= 5.1.0.

Since Drupal 6 can have a PHP version requirements in the .info file, this should be added here to forestall this problem - attached patch does this and when tested with PHP 4.4.7 prevents module activation. This requirement should also noted on the project page.