Bump minimum PHP version to 5.3.8

crypt()'s md5 is broken in PHP 5.3.7 which would cause some problems, especially with legacy password compatibility. Thanks @Arantor

Signed-off-by: Shitiz Garg <mail@dragooon.net>
Merge pull request #1713 from Dragooon/release-2.1

Bump minimum PHP version to 5.3.8