import cPickle, gzip in imdb_dataset.py (#713)
import cPickle, gzip in imdb_dataset.py (#713)

* import cPickle, gzip in imdb_dataset.py

flake8 testing of https://github.com/tensorlayer/tensorlayer on Python 3.6.3

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./tensorlayer/files/dataset_loaders/imdb_dataset.py:59:13: F821 undefined name 'gzip'
        f = gzip.open(os.path.join(path, filename), 'rb')
            ^
./tensorlayer/files/dataset_loaders/imdb_dataset.py:63:17: F821 undefined name 'cPickle'
    X, labels = cPickle.load(f)
                ^
```

* import six.moves.cPickle as pickle

zsdonghao/tensorlayer,zsdonghao/tensorlayer