[TASK] Drop support for PHP 5.3.

PHP 5.3 had its end of life on August 14th, 2014:
http://php.net/supported-versions.php

So Emogrifier now requires PHP 5.4 or greater.

Closes #79
Merge pull request #114 from jjriv/task/remove-php-5-3

[TASK] Drop support for PHP 5.3.