Use rate instead of keep_prob calling dropout in keras

While running tf.keras I noticed the following warning:
```
WARNING:tensorflow:From /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/layers/core.py:143: calling dropout (from tensorflow.python.ops.nn_ops) with keep_prob is deprecated and will be removed in a future version.
Instructions for updating:
Please use `rate` instead of `keep_prob`. Rate should be set to `rate = 1 - keep_prob`.
```

This fix fixes the warning by replacing keep_prob with rate.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>