* Pin the version of Tensorflow at >= 1.15. This allows us to remove a workaround that causes breakages for newer versions (we get errors if `AddV2` already has a registered gradient).
* Pin the version of Tensorflow at < 2 since cleverhans <= 3.1.0 doesn't properly support TF2
* Pin the version of cleverhans between 3.0.1 and 3.1.0. Versions newer than 3.1.0 will no longer support TF1.
* Comment out the dependency on `tensorflow-gpu` as it's not a strict requirement, and it does nothing for us on Travis.
PiperOrigin-RevId: 368241595
