Corrects a issue where the Login is lost (Zend-Server 6.3/PHP 5.5)

Corrects a issue where the Login is lost on Zend-Server 6.3 and PHP 5.5 and PIWIK returing to Login-Screen without message after successful login.

This is in response to Ticket #4806:
http://dev.piwik.org/trac/ticket/4806

Does regenerate the Session-ID as originally intended, however keeps the
session with the browsers resulting in a stable login on the above PHP
environment.