Add sniff for methods that changed return value

[reneoelke]

From HipChat:

• wir haben keinen Sniff/ Workflow für Methoden mit veränderten Rückgabewerten (möglicher BC-Bruch bei exakter Typprüfung; unwahrscheinlich)

From PHP migration documentation:

• http://php.net/manual/en/migration55.changed-functions.php#migration55.changed-functions.core
  -> When called with NULL, set_error_handler() and set_exception_handler() now return the previous error or exception handler, respectively.
• http://php.net/manual/en/migration55.changed-functions.php#migration55.changed-functions.intl
  -> IntlDateFormatter::getCalendar() and datefmt_get_calendar() return false if the IntlDateFormatter object was created with an IntlCalendar instance instead of one of the IntlDateFormatter constants.