Major Features and Improvements
* Updated to use TensorFlow version 1.13.1.
* Implemented Federated SGD in `tff.learning.build_federated_sgd_process()`.
Breaking Changes
* `next()` function of `tff.utils.IteratedProcess`s returned by
`build_federated_*_process()` no longer unwraps single value tuples (always
returns a tuple).
Bug Fixes
* Modify setup.py to require TensorFlow 1.x and not upgrade to 2.0 alpha.
* Stop unpacking single value tuples in `next()` function of objects returned
by `build_federated_*_process()`.
* Clear cached Keras sessions when wrapping Keras models to avoid referencing
stale graphs.