Require at least PHP 5.3.7 for security reasons

- PHP 5.3.7 introduce a fix in the BCrypt hashing algorithm.
- The BCrypt algorithm is backported (fixed in <5.3.7 on some Linux distros).
- There are several other security fixes in PHP 5.3.7.
Merge pull request #1 from Despark/php-security

Require at least PHP 5.3.7 for security reasons