Update twitter.class.php

JSON_BIGINT_AS_STRING is not supported in some linux packages.

https://bugs.php.net/bug.php?id=63520 defines the problem, and the new licence-compatible php library doesn't support this feature yet.

Change the call to check that the JSON_BIGINT_AS_STRING is defined before using it.  Fixed stock PHP (5.5.9~1) on Ubuntu 14.04 LTS.