Includes full list of available versions with GET /versions

This pull request makes the list of available versions available through the JSON REST API.

```
GET /versions
```
will now return something like:
```json
{
  "min": "154",
  "max": "164",
  "list": [
    "154",
    "161",
    "162",
    "163",
    "164"
  ]
}
```