Create backport compatible branch

[afonsocarlos]

Hi, I currently work on a project that's under PHP 5.6 version and I won't be able to upgrade it so soon. My idea and request is to create a new branch for that so we can require this library on a PHP 5.6 project. I did something roughly in this fork https://github.com/afonsocarlos/vimeo.php/tree/php-5.6

It would be nice if this repo's maintainers created such a branch for that.

Thanks.