Fix Itunes::Track#convert

# Change
- `Fix Itunes::Track#convert`
 - Use first item of converted track list in AppleScript
 - Returns a converted track instance instead of Array
- Bump up version to `0.0.5`

## Example
```ruby
track = Itunes::Track.current_track
converted_track = track.convert
=> #<Itunes::Track:0x007fa8008f3e78 @persistent_id="9ABB05FF3567B273", @name="Storytelling, Again", @album="Story Telling Again And Again", @artist="world's end girlfriend", @track_count="3", @track_number="1">
```