Json: do not use JSON_BIGINT_AS_STRING with PECL JSON-C [Closes #1244]

If JSON_BIGINT_AS_STRING option is used for json_decode(), warning arises on 64bit systems.
https://github.com/remicollet/pecl-json-c/blob/3c778e58b20ed517edb27a3937966c4b0154ab21/json.c#L977