/stemcells returns non-JSON value for deployments

```
curl -k -u admin:admin https://192.168.50.4:25555/stemcells | jazor
[
  {
    "name": "bosh-stemcell",
    "version": "993",
    "cid": "stemcell-6e6b9689-8b03-42cd-a6de-7784e3c421ec",
    "deployments": [
      "#<Bosh::Director::Models::Deployment:0x00000004696c30>"
    ]
  },
  {
    "name": "bosh-warden-boshlite-ubuntu",
    "version": "24",
    "cid": "stemcell-6936d497-b8cd-4e12-af0a-5f2151834a1a",
    "deployments": [

    ]
  }
]
```