Missing variable initialization in sample code

[trvrm]

In the article https://blog.keras.io/keras-as-a-simplified-interface-to-tensorflow-tutorial.html, I found I had to add

init_op = tf.global_variables_initializer()
sess.run(init_op)

before calling train_step.run on the MNIST data.

(I don't see a source file for this article in the content folder, so I can't send a pull request.)