Bumped minimum required PHP version to 5.3.23

Bumped minimum required PHP version to 5.3.23 to match that of package zend-form (https://packagist.org/packages/zendframework/zend-form). On very early PHP 5.3.3, Zend Form crashes with a fatal error due to Element class implementing two interfaces that both have a method with the same name (#1). PHP 5.3.3 is the latest official release for CentOS 6.5.

Also updated the minimum required version in README.md.
Merge branch 'master' of https://github.com/cronkeep/cronkeep