Specifically configure `php5-fpm` socket user, group, and permissions.

In order to workaround the bug,
introduced by `php5 (5.3.10-1ubuntu3.12) precise-security; urgency=medium`,
specify the `php5-fpm` socket user, group, and permissions.

This commit refs #86.
Merge pull request #86 from kazu634/id/86

php5-fpmソケットファイルのユーザ・グループ・パーミッションを明示的に指定する