Dropped Convert::toJSON()

In an effort to only maintain compatibility with the latest version of PHP (currently the 5.5 branch) I dropped the sanity checks if `json_encode` was available as it is always available in PHP 5.2+. Dropping this sanity check also allowed me to remove the wrapper function and the `JSON_AVAILABLE` constant. Ideally I'd like to move towards dropping the `Convert` class entirely but will need a way to convert an array to XML as the `RSS` class still leverages it. One thought is to move that code right into the `RSS` class as it never gets used elsewhere because XML is gross.