Import keras_preprocessing, not keras.preprocessing.

[mepc36]

The Keras library was split into a multi-repo in its 2.2 release, according to the Keras docs here:

The modules applications and preprocessing are now externalized to their own repositories ([keras-applications](https://github.com/keras-team/keras-applications) and [keras-preprocessing](https://github.com/keras-team/keras-preprocessing)).

Even though there is an explicitly locked dependency version in ~/requirements.txt, it looks like the code that actually does the import in this repo was never updated to make use of it.

I was getting a cannot import name 'pad_sequences' from 'keras.preprocessing.sequence' error unless I made this P.R.'s changes. See this SOF post here for more info.

If maintainer decides that this is a dependency issue that is really the user's responsibility to solve, please feel free to close with comment.

Thanks for the repo!