Added Maximum TensorFlow version.

[ThomasCassimon]

When using NASBench101 with TensorFlow 2.1.1, I received errors saying

module 'tensorflow_core._api.v2.train' has no attribute 'SessionRunHook'

After downgrading TensorFlow to 1.13.1, the issues were resolved. I am not certain which versions of TensorFlow do work, but I know 2.1.1 doesn't.

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