RNN model pytest failure

Selftest tests/unit/test_rnn_model.py no longer passes. The issue seems to be related to the new versions of Keras/TensorFlow.
See PR #87

=========================== short test summary info ============================
FAILED tests/unit/test_rnn_model.py::test_train - AttributeError: can't set a...
FAILED tests/unit/test_rnn_model.py::test_get_pretrained_model - FileNotFound...
================= 2 failed, 38 passed, 127 warnings in 41.82s ==================