[ticket/12741] Change the mode of the socket file

Since php 5.4.29, by default, php-fpm creates the socket with the 0600 mode
instead of 0666.

PHPBB3-12741
Merge pull request #2622 from Nicofuma/ticket/12741

[ticket/12741] Functional tests on Travis fail since php update last night

* Nicofuma/ticket/12741:
  [ticket/12741] Change the mode of the socket file