title: AttributeError: 'module' object has no attribute 'image_data_format'
body: <p>I have tried to run <a href="https://github.com/fchollet/keras/blob/master/examples/mnist_cnn.py" rel="nofollow noreferrer">my first demo</a> using <code>keras</code> with <code>tensorflow</code> backend but failed:</p>
  
  <p><code>
  Traceback (most recent call last):
    File "mnist_cnn.py", line 26, in &lt;module&gt;
      if K.image_data_format() == 'channels_first':
  AttributeError: 'module' object has no attribute 'image_data_format'
  </code></p>
  
  <p>keras version: 1.2.1
  tensorflow version: 1.0.1</p>
  
  <p>How to fix?</p>