amqp: fix symbol lookup error with librabbitmq >= 0.4.0

amqp_socket_close() which we were using isn't exported anymore.
See https://github.com/alanxz/rabbitmq-c/commit/bdda7ab

amqp_destroy_connection() implicitly closes the socket, so we don't need
to bother about this if using librabbitmq >= 0.4.0

credits to @tokkee for figuring that out !
Merge pull request #2 from mfournier/amqp-symbol_lookup

amqp: fix symbol lookup error with librabbitmq >= 0.4.0