add google/cloud-core as a production dependency (#1511)
add google/cloud-core as a production dependency (#1511)


jdpedrie/google-cloud-php,chingor13/google-cloud-php,googleapis/google-cloud-php,chingor13/google-cloud-php,chingor13/google-cloud-php,jdpedrie/google-cloud-php,jdpedrie/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php