Fix Keras version (avoid bug from keras==2.1.5)

[menshikh-iv]

No description provided.

[piskvorky]

What bug? Can you add a link to the description?

[menshikh-iv]

One of keras-integration test failed with Keras==2.1.5 and tensorflow==1.3.0

https://github.com/RaRe-Technologies/gensim/blob/a781b40c49a39b406bb10054ccadc86a3af84500/gensim/test/test_keras_integration.py#L47-L73

Traceback:

ERROR: testEmbeddingLayerCosineSim (__main__.TestKerasWord2VecWrapper)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "gensim/test/test_keras_integration.py", line 60, in testEmbeddingLayerCosineSim
    similarity = dot([embedding_a, embedding_b], axes=2, normalize=True)
  File "/home/ivan/.virtualenvs/qqq/local/lib/python2.7/site-packages/keras/layers/merge.py", line 660, in dot
    return Dot(axes=axes, normalize=normalize, **kwargs)(inputs)
  File "/home/ivan/.virtualenvs/qqq/local/lib/python2.7/site-packages/keras/engine/topology.py", line 619, in __call__
    output = self.call(inputs, **kwargs)
  File "/home/ivan/.virtualenvs/qqq/local/lib/python2.7/site-packages/keras/layers/merge.py", line 155, in call
    return self._merge_function(inputs)
  File "/home/ivan/.virtualenvs/qqq/local/lib/python2.7/site-packages/keras/layers/merge.py", line 484, in _merge_function
    x1 = K.l2_normalize(x1, axis=axes[0])
  File "/home/ivan/.virtualenvs/qqq/local/lib/python2.7/site-packages/keras/backend/tensorflow_backend.py", line 3151, in l2_normalize
    return tf.nn.l2_normalize(x, axis=axis)
TypeError: l2_normalize() got an unexpected keyword argument 'axis'

----------------------------------------------------------------------

To avoid this, I limited Keras version <= 2.1.4