Check for installed pcntl extension via composer.json

Hi, 

This issue is related to https://github.com/chrisboulton/php-resque/issues/98 , https://github.com/chrisboulton/php-resque/pull/93 , https://github.com/chrisboulton/php-resque/pull/94

This is not a fix Rather a update on composer to tell the user that he would need php extension PCNTL.