Render as image

When trying to render as image (and not as text in console):

```
env = gym.make('gym_backgammon:backgammon-v0', disable_env_checker=True)
mode = 'rgb_array'
```

I get:

```
    arr = np.fromstring(image_data.data, dtype=np.uint8, sep='')
AttributeError: 'ImageData' object has no attribute 'data'
```

I solved the issue in: https://github.com/ndvbd/gym-backgammon

