Specify TensorFlow backend in README
Specify TensorFlow backend in README

If I let this default to the the Theano backend, I get:

```
Traceback (most recent call last):
  File "synthesize.py", line 91, in <module>
    pyramid_model = gram.make_pyramid_model(args.octaves, args.padding_mode)
  File "/Users/ryan/mess/2017/42/subjective-functions/gram.py", line 242, in make_pyramid_model
    level = reduce_layer(padding_mode=padding_mode)(gaussian_pyramid[-1])
  File "/usr/local/lib/python3.6/site-packages/keras/engine/topology.py", line 602, in __call__
    output = self.call(inputs, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/keras/layers/core.py", line 650, in call
    return self.function(inputs, **arguments)
  File "/Users/ryan/mess/2017/42/subjective-functions/gram.py", line 150, in fn
    return K.conv2d(K.conv2d(x, kernel_3d, strides=(2,1)),
  File "/usr/local/lib/python3.6/site-packages/keras/backend/theano_backend.py", line 1878, in conv2d
    kernel_shape = kernel.eval().shape
AttributeError: 'numpy.ndarray' object has no attribute 'eval'
```
wxs/subjective-functions