BigInteger: accomodate GMP change in PHP 5.6
Merge pull request #482 from terrafrost/gmp-56

BigInteger: accommodate GMP change in PHP 5.6

* terrafrost/gmp-56:
  BigInteger: accomodate GMP change in PHP 5.6
Merge branch 'master' into php5

* master:
  BigInteger: accomodate GMP change in PHP 5.6