Fix module import and naming in MNIST CNN example

[srkiNZ84]

Summary

With the new version of Tensorflow (2.0.0alpha), it seems that the Keras module names have changed. Updating the MNIST CNN (and possibly others might need to be updated as well) example to reflect this.

Was found after reading through: #12379

Seems quite a straightforward thing to fix, not sure why nobody has committed patches.

Tested with Python 3.6.7, Tensorflow 2.0.0a0 and CUDA 10.1

Related Issues

#12379

PR Overview

• [n] This PR requires new unit tests [y/n] (make sure tests are included)
• [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [n] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[fchollet]

For the time being, the examples in this repo should import keras rather than use from tensorflow import keras.