Variable error in preprocess_card() of Cards.py

```
def preprocess_image(image):
    img_w, img_h = np.shape(image)[:2]
    return thresh
```

should be change to 

```
def preprocess_image(image): 
    img_h, img_w = np.shape(image)[:2]
    return thresh
```
np.shape() will actually return height than width