[redhat] Fix unmarshalling of the mitigation field
[redhat] Fix unmarshalling of the mitigation field

Turns out we were never unmarshalling data with a mitigation field because it's
not a string but an object with a couple of fields:

```
$ redhat_query fetch-cve CVE-2016-8705 | jq .
[...]
  "mitigation": {
    "value": "This flaw is in the memcached binary protocol. If your client programs only use the ASCII protocol when communicating with memcached, you can disable the binary protocol and protect against this flaw by adding \"-B ascii\" to OPTIONS in /etc/sysconfig/memcached.",
    "lang": "en:us"
  }
[...]
```

This means we were skipping all CVE entries with a mitigation field.

I looked at historical data and that field isn't present in the CVEs with do
have so it was probably never a string!

Before:

```
$ redhat_query fetch-cve CVE-2016-8705
failed to decode cve response into a cve: json: cannot unmarshal object into Go struct field CVE.mitigation of type string
```

After:

```
$ redhat_query fetch-cve CVE-2016-8705 | jq .
{
  "name": "CVE-2016-8705",
  "threat_severity": "Important",
  "public_date": "2016-10-31T00:00:00Z",
  "bugzilla": {
    "description": "CVE-2016-8705 memcached: Server update remote code execution",
    "id": "1390511",
    "url": "https://bugzilla.redhat.com/show_bug.cgi?id=1390511"
  },
[...]
```

facebookincubator/nvdtools