BigInteger: accommodate GMP change in PHP 5.6

There were more PHP 5.6 changes than just this but all of the other changes are BC with the way BigInteger is using GMP and do not yield any significant speedup as can be verified using https://github.com/terrafrost/phpseclib/tree/gmp-overload-exp

More info on the PHP 5.6 changes:

https://wiki.php.net/rfc/operator_overloading_gmp