databases/php-mongo: update to 1.6.16

1.6.15 (2017-09-01)

* [PHP-1489] - Can't run the test suite against MongoDB 3.1.2 or later
* [PHP-1528] - Extension should link OpenSSL during compilation

1.6.16 (2017-09-05)

* [PHP-1529] - undefined symbol: php_mongo_asn1_time_to_time_t

(taca)