Bump minimum PHP requirement to v5.3.0

As per discussion on the mantisbt-dev mailing list the minimum version
of PHP required to use MantisBT 1.3.x has now been bumped to v5.3.0. All
major Linux distributions have been using this version of PHP in their
latest releases and it is therefore widely available and accessible to
MantisBT users. The PHP project has also ended official support for PHP
5.2.x.

By bumping the minimum PHP requirement we're now able to use many of the
newer features of PHP 5.3.x.