Replace keep_prob with rate in dropout

[stephen-hoover]

The keep_prob parameter for dropout is deprecated. We should update muffnn to the new usage. Per https://www.tensorflow.org/api_docs/python/tf/nn/dropout, "Warning: SOME ARGUMENTS ARE DEPRECATED: (keep_prob). They will be removed in a future version. Instructions for updating: Please use rate instead of keep_prob. Rate should be set to rate = 1 - keep_prob." The rate parameter was added in v1.13.