Support PHP 5.3.2+

It's not easy to build composer powered code in Travis CI server
as only PHP 5.3.2+ boxes provide required tools to do that.
Additionally, Composer itself
[supports php 5.3.2+](https://getcomposer.org/doc/00-intro.md#system-requirements).
So to make support simpler, I suggest supporting PHP 5.3.2+ too.