use plt namespace for imshow in filter_visualization.ipynb

I've got errors in the filter_visualization.ipynb notebook because of

**In [6]**
```python
    # ...
    imshow(data)
```
and
**In [7]**
```python
# index four is the center crop
imshow(net.deprocess('data', net.blobs['data'].data[4]))
```

With `plt.imshow(...)` the bug is fixed