tfjs-npy doesn't work with the latest version of TensorFlow.js anymore

tfjs-npy version: 0.0.3
TensorFlow.js version: 0.14.2

The Tensor object returned by `parse()` can't be used properly by TensorFlow.js (e.g., its Model.predict() or Mdoel.fit() methods). If you try to do that, you'll get an error.

The workaround is to get the raw data out from the Tensor returned by `parse()` and create a new Tensor from it using `tf.tensor2d()` etc.
