Tensorboard on cudnn6 container fails

When running tensorboard in our cudnn6 containers

`agmb-docker run --name -tfboard-0 -p 7100:6006 -d bethgelab/deeplearning:cuda8.0-cudnn6 tensorboard --logdir="$PWD"/train_logs/`

I am running into the following problem:

```
Traceback (most recent call last):
File "/usr/local/bin/tensorboard", line 7, in
from tensorboard.main import main
File "/usr/local/lib/python3.5/dist-packages/tensorboard/main.py", line 30, in
from tensorboard import default
File "/usr/local/lib/python3.5/dist-packages/tensorboard/default.py", line 35, in
from tensorboard.plugins.audio import audio_plugin
File "/usr/local/lib/python3.5/dist-packages/tensorboard/plugins/audio/audio_plugin.py", line 27, in
from tensorboard import plugin_util
File "/usr/local/lib/python3.5/dist-packages/tensorboard/plugin_util.py", line 21, in
import bleach
File "/usr/local/lib/python3.5/dist-packages/bleach/__init__.py", line 7, in
from bleach.linkifier import (
File "/usr/local/lib/python3.5/dist-packages/bleach/linkifier.py", line 6, in
from html5lib.filters.base import Filter
ImportError: No module named 'html5lib.filters.base'
```

This seems to be related to the version of html5lib, see https://github.com/tensorflow/tensorflow/issues/8971. This issue does not arise in our cudnn5 containers. It's probably enough to manually install the correct library version in the Dockerfile.