Update to Rome 1.5.0

Closes #40.
Add support for `media:*` tags

We now add support for the new following elements:

```
      "medias" : {
        "properties" : {
          "type" : {
            "type" : "string",
            "index" : "not_analyzed"
          },
          "reference" : {
            "type" : "string",
            "index" : "no"
          },
          "language" : {
            "type" : "string",
            "index" : "not_analyzed"
          },
          "title" : {
            "type" : "string"
          },
          "description" : {
            "type" : "string"
          },
          "duration" : {
            "type" : "long",
            "index" : "no"
          },
          "width" : {
            "type" : "long",
            "index" : "no"
          },
          "height" : {
            "type" : "long",
            "index" : "no"
          }
        }
      }
```

Note that due to a bug in Rome, we can't support yet all forms described in [Media RSS specification](http://www.rssboard.org/media-rss#optional-elements).

See https://github.com/rometools/rome/issues/190 related to this.

Closes #34.