update_log.txt

#2020-03-26
require tensorflow<2.0 because some inconsistency between keras and tensorflow 2.0

# 2020-03-18
2.0.0: update tf.Session to tf.compat.v1.Session() when using tensorflow2.0


# 2020-07-25
2.0.3: update scanpy dependency such that DESC can be compatible with scanpy>=1.4+, but still with tensorflow 1.0. please see `desc_2.0.3_paul.ipynb` for detail

# 2020-07-25
2.1.1: update scanpy dependency such that DESC  can be compatible with tensorflow 2.0. Please see `desc_2.1.1_paul.ipynb` for detail