Update hm9000_client.rb

{
  "code": 10001,
  "description": "undefined method `each' for nil:NilClass",
  "error_code": "CF-NoMethodError",
  "types": [
    "NoMethodError",
    "NameError"
  ],
  "backtrace": [
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/lib/cloud_controller/hm9000_client.rb:67:in `healthy_instance'",
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/lib/cloud_controller/hm9000_client.rb:15:in `block in healthy_instances'",
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/lib/cloud_controller/hm9000_client.rb:14:in `each'",
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/lib/cloud_controller/hm9000_client.rb:14:in `healthy_instances'",
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/app/controllers/runtime/space_summaries_controller.rb:17:in `summary'",
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/lib/cloud_controller/rest_controller/base.rb:95:in `dispatch'",
    "/var/vcap/packages/cloud_controller_ng/cloud_controller_ng/lib/cloud_controller/rest_controller/routes.rb:16:in `block in define_route'",