title: Issue with setting TensorFlow as the session in Keras
body: <p>Performing the following:</p>
  
  <pre><code>from keras import backend as K
  sess = tf.Session()
  K.set_session(sess)
  </code></pre>
  
  <p>Even though I've imported Keras and TensorFlow correctly, I get the following:</p>
  
  <pre><code>module 'keras.backend' has no attribute 'set_session'
  </code></pre>
  
  <p>Any help would be appreciated!</p>