Can't inherit abstract function CacheInterface::isAvailable()

[sonnb]

Hello,

Without define any cache_driver, one of my users get this issue while processing the video:
Fatal Error: Can't inherit abstract function PHPVideoToolkit\CacheInterface::isAvailable() (previously declared abstract in PHPVideoToolkit\CacheAbstract) - library/PHPVideoToolkit/CacheAbstract.php:22

[sonnb]

This is a known issue in PHP 5.3.0 which was reported here: https://bugs.php.net/bug.php?id=43200. In order to get rid of this issue, I believe we can safety remove the abstract method isAvailable from CacheAbstract. I also created a pull request here #61 in case you want to remove it.